If and when there's a D&D 6e or 5.5e (probably 10+ years from now)...
There's should be 5* classes:
Fighter
Rogue
Cleric/Priest
Druid
Magic-User

(*maybe 6 if you argue that Paladins should their own class instead of a subarchetype of Cleric/Priest) https://twitter.com/AcademicKobold/status/1264414051326504960

Sorcerer, Wizard, and Warlock are three main subclasses of Magic-User, selected at level 1 because they each have access to arcane spellcasting via (very) different methods.
Sorcerers: inherent/genetic/soul-infused
Wizards: years of study
Warlocks: contracts w/extra-planar beings

Ranger is a primal casting based Fighter subclass (the druidic equivalent of Eldritch Knight).

(Or an expert-explorer Rogue subclass, but that would require a robust exploration pillar reflected in the ruleset. Aka hexcrawl mechanics)

Barbarian = likely another Fighter subclass

Bards would also be a Magic-User subclass (almost forgot about them)

I'd love the subclass design (Divine Domains) for Cleric/Priest to be reworked. The Light domain is broad in superficial flavor and yet weirdly specific in its mechanics and their narrative implications.

Essentially the class breakdown becomes:
Fighter
• Champion
• Battlemaster
• Eldritch Knight (arcane 1/3 caster)
• Druidic Warrior (primal 1/3 caster)
Magic-User
• Wizard
• Sorcerer
• Warlock
• Bard
Priest
• Cleric vs Paladin archetypes?
• Domains?

Druids & Rogues, etc

At this point, I'm using this thread to think out loud...

But Paladins would a Fighter subclass under this system.

There'd be a Fighter subclass for every kind of caster (Arcane, Primal, Divine)

Eldritch Knight
Druidic Warrior (aka Ranger)
Paladin / Holy Knight

Rogues could have this, too.

Arcane Trickster

Infernal Agent
• basically the Divine caster Rogue, except more Warlock-like and probably still an arcane caster but w/cleric spells that devils would use

...a Rogue-ish archetype that fits with nature magic?