The standard ability scores used for spellcasting by various classes broadly make sense--an archetypal Cleric ought to be wise and an archetypal Bard ought to be charismatic--but they leave something to be desired when it comes to nonstandard character concepts: the enthralling preacher Cleric, or the bookish archivist Bard. However, I'm wary of allowing people to mix-and-match classes with ability scores, because doing so can cause some balance issues vis-a-vis multiclassing. So, here's a compromise:
A spellcaster may choose, at first level (or the level at which they get their Spellcasting or Pact Magic feature), to use an alternate ability score for their magic. This is intended as a flavor decision, not an increase in mechanical power: a character who uses an alternate spellcasting ability score may not multiclass without specific DM approval. Approval will be granted only if the alternate ability score has no impact on the multiclass combination in question, such as a Charisma Cleric multiclassing with Battlemaster Fighter. If you use an alternate ability score for your spellcasting, all other class features that use an ability score also use your new spellcasting ability score.
The combinations described below (which include the class's default casting ability score) are allowed no questions asked, but if there's a different class/ability score combination you'd like to use you can feel free to run it by me. If it makes for a cohesive character concept, and it makes sense that your character's magic would be tied to the ability score you choose, I'll probably allow it.
Artificer. Intelligence
Bard. Intelligence, Charisma
Cleric. Wisdom, Charisma
Druid. Wisdom
Monk. Wisdom or Charisma (While not spellcasters, Monks are overtly magical and heavily reliant on Wisdom.)
Paladin. Wisdom or Charisma
Ranger. Wisdom
Sorcerer. Charisma
Warlock. Intelligence or Charisma
Wizard. Intelligence
Sorcerers may use the Spell Points variant rule from the Dungeon Master's Guide (p.288). Sorcerers no longer have a separate pool for sorcery points; rather, add your sorcery points to your pool of spell points. If you later take levels in another class with the Spellcasting feature, you must track your spell slots for that class separately from your Sorcerer spell points. So, a 5th level Sorcerer/1st level Bard has 22 spell points (17 plus 5 sorcery points) and 2 spell slots.
Tasha's Cauldron of Everything contains two Sorcerous Origins (Clockwork Soul Aberrant Mind) that receive additional spells known. To bring the Sorcerer subclasses published before Tasha's Cauldron into line with the newer, more powerful ones, each Sorcerous Origins receives one additional class feature at 1st level granting it more spells known (see below). Divine Soul is the exception; its Divine Magic feature is instead replaced by the one below.
Divine Soul - Divine Magic
You learn additional spells when you reach certain levels in this class, as illustrated below. Each of these spells counts as a sorcerer spell for you, but it doesn't count against the number of sorcerer spells you know.
When you gain a sorcerer level, you can replace one spell you gained with this feature with another spell of the same level. The new spell must be from the cleric spell list.
Additionally, any time you learn a sorcerer spell, you may choose a spell from the cleric spell list or the sorcerer spell list. It counts as a sorcerer spell for you.
1st level - bless, detect evil and good
3rd level - aid, augury
5th level - remove curse, speak with dead
7th level - banishment, guardian of faith
9th level - commune, greater restoration
Draconic Bloodline - Dragon Magic
You learn additional spells when you reach certain levels in this class, as illustrated below. Each of these spells counts as a sorcerer spell for you, but it doesn't count against the number of sorcerer spells you know.
When you gain a sorcerer level, you can replace one spell you gained with this feature with another spell of the same level. The new spell must be an enchantment or an evocation spell from the sorcerer, warlock, or wizard spell list.
1st level - absorb elements, cause fear
3rd level - darkvision, dragon's breath
5th level - fear, protection from energy
7th level - elemental bane, polymorph
9th level - control winds, dominate person
Shadow Magic - Umbral Sorcery
You learn additional spells when you reach certain levels in this class, as illustrated below. Each of these spells counts as a sorcerer spell for you, but it doesn't count against the number of sorcerer spells you know.
When you gain a sorcerer level, you can replace one spell you gained with this feature with another spell of the same level. The new spell must be an illusion or a necromancy spell from the sorcerer, warlock, or wizard spell list.
1st level - false life, inflict wounds
3rd level - darkness, shadow blade
5th level - feign death, summon shadowspawn
7th level - death ward, shadow of moil
9th level - enervation, negative energy flood
Storm Sorcery
You learn additional spells when you reach certain levels in this class, as illustrated below. Each of these spells counts as a sorcerer spell for you, but it doesn't count against the number of sorcerer spells you know.
When you gain a sorcerer level, you can replace one spell you gained with this feature with another spell of the same level. The new spell must be an evocation or a conjuration spell from the sorcerer, warlock, or wizard spell list.
1st level - feather fall, fog cloud
3rd level - gust of wind, skywrite
5th level - thunder step, lightning bolt
7th level - storm sphere, control water
9th level - control winds, maelstrom
Wild Magic
You learn additional spells when you reach certain levels in this class, as illustrated below. Each of these spells counts as a sorcerer spell for you, but it doesn't count against the number of sorcerer spells you know.
When you gain a sorcerer level, you can replace one spell you gained with this feature with another spell of the same level. The new spell must be an abjuration or an evocation spell from the sorcerer, warlock, or wizard spell list.
1st level - chaos bolt, chromatic orb
3rd level - mirror image, nystul's magic aura
5th level - counterspell, dispel magic
7th level - confusion, otiluke's resilient sphere
9th level - bigby's hand, reincarnate