Materials from almost all officially published WotC material are available in some setting somewhere, though some have changes listed in these rules. All UA material, Stream content, and any features that are setting-specific require DM approval and possible modification before using.
Note that most settings' character creation rules list specific sourcebooks for allowed materials. This is not a full ban on materials outside of those sourcebooks; it just indicates that those sourcebooks have no relevant material that conflicts with that setting's lore. There is a better than average chance that materials from sources not listed would be allowed if you ask, unless they are from a setting-specific book.
Currently, the only allowed material in all settings from the Astral Adventurer's Guide is the Air Bubble spell.
Items not from official WotC material require heavy DM scutiny to be accepted, and may be reworked on the fly.
Currently accepted homebrew material for most settings (only disallowed if explicitly banned in that setting's Allowed Materials section):
Matt Mercer's Blood Hunter + Lycan subclass
KibblesTasty Psion
KibblesTasty Elemental Spells
Way of the Four Elements Remastered
My all-settings homebrew material
When you make a spellcasting character, multiclass into a spellcasting class, or otherwise gain the ability to cast spells from a class or feat, you may request for the class' spellcasting ability score to change to a different one, if you can give a solid justification for it. For example, you may decide that your warlock gained their power through studying forgotten research of an eldritch entity, and thus uses Intelligence as their casting stat. Once you start playing this character as a spellcaster, you may not change their casting stat.
If you are starting this character from level 1 with a different casting stat, you may change their starting saving proficiencies if they do not normally start with proficiency in saves from their new casting stat. All characters must have one proficiency from the 3 more common saves (Dex, Wis, Con) and one proficiency from the 3 less common saves (Int, Cha, Str), so the save proficiency you replace must be in the same tier as your new casting stat.
If you have a companion creature that you control and that creature rolls initiative at the same time as you, you can choose whether that creature rolls initiative for itself or acts on your turn.
If you have a class feature that says you can magically conjure or summon a weapon in your hand, it counts as a free object interaction, similar to actually drawing a weapon.
Each point of exhaustion grants a cumulative -1 penalty to attack rolls, ability checks, and saving throws. This stacks up to 9 points, for a maximum penalty of -9. If a creature reaches 10 points of exhaustion, they die.
All creatures' fall damage caps at 200ft (20 dice), but the damage dice change based on the creature's size - the bigger you are, the harder you fall.
Miniscule creatures take no damage from falling
Tiny creatures take 1 damage, to a maximum of 20 points
Small creatures take 1d4 damage, to a maximum of 20d4
Medium creatures take 1d6 damage, to a maximum of 20d6
Large creatures take 1d8 damage, to a maximum of 20d8
Huge creatures take 1d10 damage, to a maximum of 20d10
Gargantuan creatures take 1d12 damage, to a maximum of 20d12
Colossal creatures take 1d20 damage, to a maximum of 20d20
If your speed is not 0 when you land, you can make a DC 10 Dexterity saving throw to reduce your effective fall distance by 10ft and land upright instead of prone. If you succeed on this saving throw by 5 or more, then you reduce the falling distance by an additional 10ft for each 5 you beat the DC by.
For the purposes of resistance/immunity, falling deals nonmagical bludgeoning or piercing damage, depending on the surface you fall onto.
If a Tiny or larger creature or object falls onto another creature or object, half of the fall damage taken by the falling creature or object is also dealt to the target. A creature can make a Dexterity saving throw to avoid the damage. The saving throw DC starts at 10 for a Tiny falling entity, and increases by 2 for each size category it is above Tiny, up to a maximum of 20 for a Gargantuan or Colossal entity.
If you attempt to weaponize this, note that objects and creatures falling from high up may drift unpredictably due to environmental factors, and rolls for aiming may be required.
Alternatively, if a creature falls on their turn, they can attempt to make a melee weapon attack against a creature or object on the ground where they land. Instead of a saving throw from the target creature, the falling creature must make an attack roll with a -1 penalty to hit for each 40ft fallen, to a maximum of -5 (200ft). If the attack misses, the falling creature takes full fall damage and lands prone. If the attack hits, the falling creature takes half damage from the fall, and the other half is added as bonus damage to the attack. A creature can only do this once per turn.
Creatures and objects normally fall at a rate of 500ft per round. For falls less than 500ft, the fall is usually treated as happening nearly instantly. However, if a creature falls on their own turn, they can use their Action, Bonus Action, or Reaction at any point during the fall. Similarly, if you throw, drop, or otherwise cause a creature to fall on your turn, you may use an Action, Bonus Action, or Reaction at any point during the creature's fall.
If a Paladin breaks their oath, a Cleric loses the favor of their God, or a Warlock breaks their contract, the player may have those powers stripped away. This may be handled on a case-by-case basis, but barring any more interesting story options, the default punishment is this:
All the character's levels in the relevant class will be replaced by one level of a "Fallen" class. The "Fallen" class will include every nonmagical class feature the character had gained up until that point, including HP and Hit Dice, but not including Proficiency Bonus. Subsequently gained levels in other classes will not grant new HP or Hit Dice until the character has surpassed the character level at which they fell, nor will they grant ASIs until the new classes' ASIs outnumber the ASIs they had under the first class. If the character does not already have levels in any other classes at the time they fall, they do not need to meet the ability score prerequisites to multiclass into any other class, but must still have a valid roleplay reason to take the new class.
Certain settings have specific rules for firearms. In Voidspace, as well as in any setting that does not specify specific rules for firearms, the following rules apply:
Firearms use the stats from the DMG. Most settings would only have up to Renaissance firearms (Pistol/Musket), but more technologically advanced settings may have Hunting Rifles, Shotguns, and Revolvers. Primespace is the only setting with Automatic and Futuristic weapons.
Firearms have a chance to misfire on a low roll. Each firearm has a Misfire Score equal to the base number of damage dice the weapon deals. When an attack roll using that firearm rolls a number on the d20 at or below the weapon's Misfire Score, the gun jams, and cannot be used again until an action is used to clear it.
Any feature that allows you to count as one size larger for the purpose of determining carrying capacity, such as Brawny or Powerful Build, also allows you to count as one size larger for the purposes of grappling and throwing.
When you Ready an Attack action, your reaction attack benefits from the Extra Attack feature.
When rolling a hit die on a level up, if the die lands on a 1, you may reroll it until it lands on anything else. Alternatively, you can take the average roll.
If you want to multiclass, please let the DM know before you level up, so that plot hooks can be made available for you. You will need a roleplay reason to want to multiclass, and another roleplay excuse for how you will gain the new class features. If you multiclass into any "learned" class, you will need to spend at least some downtime training for your new class features. In the time directly leading up to and after your multiclass, you may gain the level one features slowly or at specific moments as they become relevant through roleplay. In this case, you would only gain the Hit Points and Hit Dice from your multiclass when you actually level up.
The multiclass prerequisites can be modified or ignored depending on your primary ability scores for your build. For example, you do not need 13 Str to multiclass as a paladin if you use Dexterity for all your attack rolls, but you would need 13 Dex. Personally, I am much more lenient with the prerequisites to multiclass out of a class than the prerequisites for multiclassing into a class.
If you are at least one size category larger than an inanimate object or structure and you hit it with a melee attack that deals bludgeoning damage, the damage is doubled.
Drinking a potion in combat is a bonus action. Administering a potion to another character is an action, and requires that the target character must be either willing or incapacitated.
Alternatively, you can attempt to throw a potion at a creature as an improvised weapon attack. If the target is a willing creature, the attack has advantage. On a hit, the potion takes effect as if the target consumed it.
One-handed Light ranged weapons can be wielded in the off hand, and a bonus action can be used to attack as if using the Two Weapon Fighting mechanics.
The ranged weapon does not have to be Light if the wielder has the Dual Wielder feat, but the +1 to AC will not apply unless both weapons are melee weapons.
Short rests require 1 minute of rest in a non-threatening situation.
There are two new homebrew sizes with unique properties: Miniscule and Colossal
Miniscule: One size category below Tiny. Miniscule entities are incredibly small, and can fit through gaps 1 inch wide without squeezing. Creatures this size have their base carry capacity halved twice, and do not take or deal damage from falling. This size category is intended for insects and similarly-sized creatures/objects.
Colossal: One size category above Gargantuan. Creatures, Vehicles, and other Objects that take up spaces larger than 30ft x 30ft are considered Colossal. Colossal entities cannot be grappled except by other colossal entities, and on a battlemap are treated more as terrain than as an actual singular entity. They usually follow the same action economy rules as other creatures and vehicles, but generally have different sets of hit points for different parts of them. For example, a colossal sailing ship might have a separate HP pool for its hull and sails (or even separate HP pools for separate segments of its hull), or a plane may have separate HP pools for its wings and fuselage. The largest of creatures and vehicles may even have different hit point pools for different Large or Huge sections of a single segment.
Spell Scrolls can be used by any character regardless of spellcasting. If it is not a spell from your class' spell list or of a level you cannot cast, you must roll an Arcana check of a DC equal to 10 + the spell's level in order to successfully use the scroll.
When you grapple a creature that needs to breathe, you can attempt to strangle it.
If the creature is Surprised, you can initiate the chokehold as part of the action used to grapple it. The creature must make a Con save against a DC equal to (8 + your Str mod + your prof bonus) or run out of air instantly, following the below suffocation rules from there.
If the creature is not Surprised when you first grapple it, or succeeded on its Con save while it was Surprised, you need to use an Action to start strangling it after you already have it grappled. In this case, the creature is considered to be holding its breath. When you initiate the chokehold, and each time you use your Action to maintain it, the creature must make a Con save against a DC equal to (8 + your Str mod + your prof bonus) or lose half its remaining air.
In either case, you must use your Action every round to maintain the chokehold until they drop to 0 HP. If you do not use your Action to maintain it, the creature can breathe again at the end of your turn. The creature also is able to breathe again if the grapple ends.
You can hold your breath for a number of minutes equal to 1 + your Con modifier (minimum 30 seconds, or 5 rounds). Every time you take damage while holding your breath, the number of rounds you have left is halved (rounding down). You immediately lose all your breath if you cast a spell using Verbal components, activate a magic item with a command word, or you are Incapacitated.
If at the end of your turn you have no air left and attempt to continue holding your breath, you must make a Constitution saving throw. The DC starts at 10, then increases by 2 with each successive round without air. On a failure, you drop to 0 HP, becoming Unstable, and you cannot regain HP until you can breathe again.
The Surprised condition wears off after one full round has passed, instead of on a surprised creature's first turn.
When in a non-stressful situation where you can repeat a skill check for an arbitrary amount of time, you can choose to use your passive skill in place of an active skill check. For example, this can apply to searching a room for hidden doors or compartments, studying a topic in a library over downtime, or crafting an item that you have crafted before.
If you would have advantage on the check, your passive score increases by 5 for that check, and vice versa for disadvantage.
If a creature you are grappling is your size or smaller, you can attempt to Throw that creature as an action. If the target is unwilling, make an Athletics check, contested by the target's Athletics or Acrobatics. If you succeed (or if the target is willing), you can throw the creature a number of feet horizontally equal to your strength modifier × 5 (minimum 5ft), or half that distance vertically.
If a thrown creature hits an inanimate obstacle before reaching the maximum throw distance, both the creature and the obstacle take 1d6+Str bludgeoning damage for every 10ft the creature traveled before impact. If a creature is thrown towards another creature, the thrower may make an improvised ranged weapon attack. On a hit, both the thrown creature and targeted creature take 1d6+Str bludgeoning damage.
This is a full action, and cannot replace an attack in the Attack action unless you have the Grappler feat.
Barbarian:
Frenzy (lv3 Berserker): While you are in a frenzied rage, your strength-based attacks deal extra damage equal to your current points of exhaustion.
Bard:
Superior Inspiration (lv20): If you have spent one minute with no uses of bardic inspiration left and no other creatures having inspiration granted by you, you regain one use of bardic inspiration.
Druid:
Archdruid (lv20): You do not have infinite uses of Wild Shape. However, if at the start of your turn you have no uses of Wild Shape remaining, you regain 1 use of Wild Shape.
Fighter:
Indomitable (lv9): When you reroll a save using this ability, you have a bonus to the roll equal to your fighter level.
Monk:
Martial Arts (lv1): Monk Weapons count as Finesse, including Unarmed Strikes.
Perfect Self (lv20): If you have less than 4 ki points remaining, you can spend an Action to center yourself and regain 1 ki point.
Ranger:
Ranger's/Primal Companion (lv3 PHB Beast Master): Your beast companion has its own full turn and doesn't need you to spend any actions for it to take whatever action you command it to, though it loses multiattack (if it has one) until level 11.
Exceptional Training (lv7 PHB Beast Master): You can command your beast companion using your Bonus Action. Your beast companion can immediately use its Reaction to take the Attack, Dash, Dodge, or Help action.
Sorcerer:
Spellcasting (lv1): You may use the Spell Points system instead of Spell Slots for your sorcerer Spellcasting. In addition, you may now Prepare spells from the Sorcerer spell list equal to the number in the Spells Known column in the Sorcerer class table.
Origin Spells (lv1): If your sorcerer subclass does not have a feature at level 1 giving you bonus spells, you gain the Origin Spells feature, giving you more spells from 1st to 5th level based on your specific subclass. You always have your Origin Spells prepared, and they do not count against the number of spells you can prepare. See the All Settings Homebrew materials for the lists of these spells.
Metamagic (lv3): Instead of learning metamagic options permanently, you can prepare 2 metamagic options from the list of available ones at the end of a long rest. This increases at 10th and 17th level, in line with where RAW sorcerers learn more options.
Warlock:
Hex Warrior (lv1 Hexblade): This feature does not allow you to use Charisma for weapon attacks and damage.
Pact of the Blade (lv3): You can use your Warlock spellcasting ability for attacks and damage rolls with your Pact Weapon.
Dual Wielder: If you can magically conjure or summon one weapon from a class feature, you can now conjure or summon two non-heavy weapons at once.
Grappler: When you take the Attack action, you can replace one of the attacks with an attempt to Throw a creature.
Heavy Armor Master: Damage reduction applies to magical weapon damage as well.
Tavern Brawler: With this feat, your Unarmed Strikes have the Finesse property.
Close Quarters Shooter: Does not grant a bonus to ranged attack rolls.
Mariner: Explicitly banned
Protection: Instead of imposing disadvantage, you can redirect the attack to be against yourself.
Tunnel Fighter: Explicitly banned, see rework in all-settings homebrew
Blade Ward: Casting time of 1 reaction, force disadvantage on a melee attack roll against you.
Control Flames: Can be used on magical flames created by the caster or under the caster's control.
Guidance: Range increased to 10ft, duration changed to instantaneous, casting time changed to 1 reaction (which you take when an ally in range fails an ability check).
Resistance: Range increased to 10ft, duration changed to instantaneous, casting time changed to 1 reaction (which you take when an ally in range fails a saving throw).
True Strike: Casting time of 1 bonus action, not concentration.
Ceremony: It is possible to use the Ceremony spell for more than just the listed uses, though the benefits (and risks) depend entirely on DM interpretation of the situation. This may include hallucinatory missions, along the lines of a 'spirit quest' constructed for the target by the caster.
Silvery Barbs: 2nd level spell with no advantage, can upcast to give one creature advantage per slot level above 2nd.
Find Familiar: Familiar can take the form of any Tiny CR 0 creature.
Witch Bolt: The repeat damage is scaled with higher level spell slots.
Find Steed: Steed can take the form of any Large or Medium Beast of CR 1 or lower, with no Fly or Swim speed, that the caster has seen before.
Find Greater Steed: Steed can take the form of any Large or Medium non-bipedal creature of CR 2 or lower that the caster has seen before.
Find Traps: All trigger mechanisms for traps you can see within range glow until the end of your turn.
Dispel Magic: Can only dispel one spell per cast. You must specify what spell you are dispelling (you do not necessarily need to know exactly what the spell is, only that it's there). Also may be able to dispel other magical effects or properties, with a DC dependent on DM judgement.
Sending: You must have at least seen the person you are trying to contact and know at least one name they have gone by, though you need not have met them in person. Additionally, if you use the Sending spell to contact someone who knows you by a fake identity, they interpret it as coming from the identity they know. If the recipient knows multiple identities belonging to the caster but does not know that they are the same person, the caster can decide which identity the Sending spell will use.
Stoneskin: No material cost; reduces magical damage as well as nonmagical. Does not reduce damage from Adamantine weapons or weapons with the Siege property.
Mordenkainen's Sword: No concentration, movement increased to 60ft / round, two attacks per round