Ability Scores. Standard array (15, 14, 13, 12, 10, 8) or 27-point buy are both fine. (A handy point buy calculator can be found here.) If you would rather roll for your ability scores, I'll permit 3d6 rerolling ones once--so long as you roll in Roll20 where I can see. (4d6 drop lowest can be a lot of fun, but I prefer player characters that stick closer to the default assumptions of the system--and 3d6 rerolling ones once results in an average score almost identical to point buy.)
Races. While the Western Isles have relatively specific demographics, all published races should be alright. If you want to play something other than a human or halfling, you might want to take a look at their description under People of Harth--and if you have any questions about a race's place in the setting, don't hesitate to ask. As per Tasha's Cauldron, regardless of your choice of race, you may place your racial ASIs wherever you like (though they may not overlap). Custom Lineages from Tasha's are also permitted, so long as they're thematically cohesive.
Classes. All published classes (including Artificer) are permitted, as are all published subclasses for them. As per Tasha's Cauldron of Everything, we will be ignoring race-based restrictions on subclasses like Bladesinger and Battlerager. You are free to play any class as-written, however I have a few houserules for certain classes (mostly those I consider to be underpowered) that you are welcome to use (see Changes to Classes).
Backgrounds. Custom backgrounds are of course allowed (they aren't even a variant rule!), but if you want to use a background feature from a non-PHB background, please let me know--I'm not familiar with all of them, and a few are quite plot-significant.
Multiclassing. Multiclassing is permitted without caveats, however minimum ability scores are enforced.
Feats. Feats are permitted. Because I'm not a fan of racial requirements, most racial feats from XGtE that make no reference to a pre-existing racial trait have had their racial prerequisites removed. Thus, Drow High Magic, Dwarven Fortitude, Elven Accuracy, Fade Away, Fey Teleportation, Prodigy, Second Chance, and Wood Elf Magic are all available to characters regardless of race (although Drow High Magic, Fey Teleportation, and Wood Elf Magic are all highly cultural and probably demand some roleplay explanation if you aren't the appropriate type of elf).
Alignment. I don't like alignment. You're free to choose an alignment for your character if you like, but you don't have to, and if you do it'll only matter insofar as it affects how you play your character. "Evil" characters are fine, but unhinged villain types (or other characters with overtly destructive motivations) are not suitable player characters, nor are characters who don't want anything to do with the other player characters. Playing a villainous loner can be a lot of fun, but it's much harder to run a satisfying game for a party full of them.
Sourcebooks. Character options from all published first-party books (Player's Handbook, Dungeon Master's Guide, Sword Coast Adventurer's Guide, Volo's Guide to Monsters, Xanathar's Guide to Everything, Mordenkainen's Tome of Foes, Tasha's Cauldron of Everything) are permitted, as is content from the Elder Evils Players Companion and Ebberon: Rising From the Last War. Critical Role content is also acceptable, but other unofficial content is very likely not (though you can always ask).
Unearthed Arcana. Some UA content is excellent; some is badly broken, terribly underpowered, or wildly incongruous with the rest of the game. I will be reviewing UA content on a case-by-case basis: if there's a UA feature you'd like to use, please ask! I'm generally pretty permissive.
Homebrew. In general, homebrew (other than my own) is disallowed. However, I'm pretty receptive to minor tweaks if appropriate to the character and with minimal game balance consequences. If there is a specific, well-balanced piece of homebrew (subclass, feat, spell, fighting style, etc) that you didn't personally create, I'll take a look at it, especially if it's very similar to published content that already exists. Homebrew races and classes are a hard no, though--finding a way to fit a new homebrew race into a setting takes considerable effort, as does reading an entire homebrew class and making sure it isn't broken.