On his Patreon, Mike Mearls talks about how the Class + Subclass is one of the simpler ways of doing 5E class. To prove his point, he's currently designig a new class, the Psyker, that has menus of features throught their levels (kind of like PF2E feats), and choices on certain menus can impact non-menu features later on in the class. It gives you a lot of choice, is still relatively simple, but doesn't work with 5E multiclassing -- something Mearls says is tech debt, but he's working on a new kind of multiclassing to address this.
These advanced classes with menus at every odd level are, IMO, something that could greatly benefit the game. You could have Sidekick Classes, Classes with Subclsses, and Classes with Menus and really just have a lot of fun creating different fantasies via new Class options. Even some 5E options, like the Warlock or Ranger, would IMO work better as Menu Classes (I'm working on a Menu Ranger right now, actually).
Ultimately, I feel like class balance is held back by Multiclassing and the huge amount of generic spells. If there were fewer spells, or if spell lsits were smaller and more diverse, there would be no problem adding more classes. I think these two changes (more diverse spell lists and three paradigms of Class design) would turn 5E into the true Ur-Fantasy game, one easy to make content for and easy to use for your exact table's whims. Likewise, with more Alternative Class Features, you could do so much, like making Alternative features for more Social or Exploration-themed campaigns.
If WotC just truly embraced the toolbox game kit idea, D&D could just be so much more for EVERYONE who plays it imo