Either the sorcerer or a new class - it depends on how many classes the hypothetical 6e has.
Alternately, rebuild all the classes with a simple chassis that you can then optionally add complexity to. Though that would be a fairly radical reworking of the game, I think.
I fully agree with everything here.
And speaking about radical reworking: 3E, 4e and 4e were all radical reworkings compsred to the previous edition.
Also game design (especially outside rpgs) evolved a lot since 5e released, so I dont see this as so impossible.
Also if you think about the suits st wotc/hasbro.
D&D is s 100 million dollar (per year) franchise. So next step would be a billion dollar franchise like League of Legends, Overwatch, etc.
So I could see some suits looking at League of legends with 300 million players and even wanting "moba like classes".
So classes having 1 or 2 basic attacks (secondary weapon) which would be weapon attacks with weapon mastery or cantrips.
1-3 abilities you can use once or twicr per combat.
1 ultimate ability which you can only use once every phew fights.
And a passive.
However, if we dont look at moba/hero shooter design. Having simple chassis is not that impossible.
5.24 could not change subclasses to level 1 because thats too complicated so they made recommended starting level 3.
In a 6E everyone could get their subclass at level 1 and make level 1 in 6e equal to level 3 in 5e.
There might be a tutorial level 0 before for people new to the game, but making the desired starting level to level 1 makes it easier for everyone.
Now how they could not standartize the subclasses in 5e because of the different base class power levels, and had hard tome making good subclasses for casters because of their loe subvlass power budge, it would make sense to make the base classes really simple (as I proposed before) and have a lot of power and potential compleity come from the subclass.
We have in 5e already the martials which gain spellcasting over subclass. So they know this works so no reason to not try this also with casters.