Dragon Sorcerer Level 18 Capstone: You Become an actual Adult Dragon of your origin. No kidding. All Full casters can turn into Dragons by that Level, but Dragonic Origin Sorcerers can't and that's silly. I'd include the Shapechange ability even to Chromatics, to turn back into their previous Selves, and not include Legendary actions... Probably also Keep their own Mental Abilities (for better or for worse) in both shapes... That might actually encourage a sorcerer to play up to level 18, instead of multiclassing into Paladin (or playing a Wizard for an Arcane Caster - I don't, but most people do so).

Totem Barbarian: Remove the spellcasting restriction on rage... it's enough that you can't concentrate on spells. The Totem Barbarian/Shaman is the "Spellcaster" Barbarian, and should be able to use some Raging Spellcasting... I'd love the idea of some Raging Eldritch Blasts as a theme (without it necesserally giving any benefit whatsoever, just for the idea of an RP trope).

Eldritch Knight; War Magic: If only if worked the other way around! Either:
Option 1: When you make the Attack action on your Turn, you can use a bonus action to cast a cantrip.
Option 2: When you use your action to cast a cantrip, you can use a bonus action to use the Attack action.

Or, the perfect one:
War Magic: When you use your action to use an Attack Roll Cantrip, you can use a Bonus action on your turn to use one more Attack Roll Cantrip on your turn.

Walock, Pact of the Blade: You use your Charisma Mod for the Attack/Damage Rolls of the Blade, instead of Str/Dexterity. The Blade is made of pure eldritch energy unless you make a specific weapon your pact weapon (fluff change).

Paladin, Oath of Vengence: Darker tone to the actual oath, making him more of a neutral Paladin, with Evil tendencies, than a Chaotic Good Paladin. Fluff Changes.

Paladin/Ranger: Give them some cantrips! The only reason I'd never consider playing a Pure Paladin/Ranger is because they have no cantrips. Real Casters have Casntrip, and Even Arcane Tricksters/Eldritch Knights feel more like real casters to me, than Paladins or Rangers (eventhough they have less spells/spellcasting progression) because of this fact.