It depends a bit on how many classes 6E would have.
If its just 13 or so, then the sorcerer, because that one is in general the one least distinct from the wizard.
If we think about harry potter (who is today highly associated with wizard and who knows like 3 spells or maybe 5) it could also be the wizard, but I think that would annoy old players.
If there are a bit more classes, then I would reintroduce the elementalist from 4E. I think an element based blaster caster with inborn talent and not many different spells is really fitting.
Also if we have subclasses similar to 5e but from level 1, then I would do several (if not all) base classes simple and put the complexity behind subclasses (give them a bigger power budget).
Like the sorcerer would only get 2 cantrips and sorcery points (but low number but they are more impactfull per point) in the main class. With 1 use for sorcery points (lets say upgrading cantrips)
And then the elementalist subclass would give them upgrades to the cantrips basic elemental features and improve the upgrade to cantrips.
A wizard would get cantrips and 1 signature spell they could cast X times per day at highest level in base class.
A simple subclass would improve cantrips and the signature spell and allow a 2nd one.
A warlock would only get eldritch blast (or eldritch strike) + 1-2 other cantrips + hex as a class feature.
A simple subclass would also improve cantrips and hex.
And then the "normal" subclasses would introduce full casting to these classes.
To not piss off old people, one could make it a bit more like PF1s archetypes.
Like you have the main class description in the book include the "most signature" subclass in its writing, with subclass features marked with a star. One of them being full casting. And subclasses then replace these features like archetypes in Pathfinder 1.
here an example from the PF1 inspired Final Fantasy D20:
Iaijutsu Master (an example of a subclass replacing the base classes main ressource feature).
Or here is one which reduces a full caster to a 3/4 caster:
Muscle Mage