Honestly, one of the issues I see with D&D is too much class variety leading to balancing nightmares. Maybe just make a class called wizard and they can cast anything, then give them benefits depending on how deeply they focus in a specific area. For example, improve their spell modifiers based on how many of a spell type they have meditated on and put in their spellbook? So if you want to be an effective healer, you need to have primarily healing spells learned (makes sense, that's what you studied on the night before). If you want to be a nuker for the day, you need to study primarily evocation.
The issue I see that arises out of how D&D currently works is that you're forced to bring along a healer, a tank, and a rogue. So immediately your party selection is reduced by 3. In a game (tabletop or cRPG) where you only have a party size of 4 (engine limitations or social limitations), that gives you 1 option for creativity and roleplaying. Then in order to counterbalance this issue, you get ranger pets as semi-tanks, healing potions & scrolls of resurrection that demean and cheapen the seriousness of high level magic, summoned creatures that need to be strong enough to tank NPCs, but not strong enough that they nullify the reason for a player to pick a tank, etc. etc. etc. This all restricts your options for roleplaying and game design IMO. I personally liked Larian's take on leveling in DOS1/2 and how you could easily alter a starting NPC by leveling up different things for them than they originally started with.
