Everybody has a different sweet spot for this. In my ideal world, we'd wipe out the difference between arcane, primal, and divine and just have "magic." Spells would be cast with a "Spellcasting" skill (or skills). And any Spell-like abilities that a character had that was more reliable would be something that required expending a character advancement resource to obtain.
At most, you'd need 2 or 3 "classes" (warrior, mage, and skill monkey), and everything else would just be a combination of those. Or, better yet, at that point, we ditch the class system and just make everything skill-based. The older I've gotten, the more I have come to see the class system as a bug, rather than a feature. I still like the idea of archetypal characters, but I have come to think classes are a bad way of implementing it. It's especially true in a genre where your average warrior hero is typically way more than just a strong, dumb tank.
Of course, I also want to overhaul the damage system and how armor works, because it really bothers the trained swordsman in me. I don't want "realistic" combat, I want something that's fast, playable, but makes it easy to narrate the flow of a fight. And we need a better system for stunts and special maneuvers. Think "appropriately cinematic" combat and action sequences.
But then, that probably wouldn't really be D&D to many people. I could shoehorn the subsystems in as 5e SRD mods, but it's starting to feel like hammering a square peg into a round hole. But I might do it anyway.