My perfect version of D&D would probably steal mechanics heavily from 4E, 5E, and Worlds Without Number, as well as a few other systems, like key things it needs are:
1) Combat that at least can be TtoM without being wrecked.
2) Encounter-based design and balancing, rather than attrition-based design.
3) Heavier use of ritual magic.
4) Monsters that are defeated through knowing their weaknesses more (hopefully without just making them immune to everything).
5) No power gap between casters and others, and none of this "start weak become mega-OP" nonsense (which explicitly the design of OD&D, note)
6) Not species-as-class but stuff like vampire-as-class? Go for it. If it's a thing you can be transformed into, cool.
7) Multiclassing where you're actually both classes (probably 2 class max MC'ing, three classes was always shenanigans), you're not just taking a level in X. I think this would actually be easier to balance, long-term, as it entirely avoids "dips" and similar nonsense. Some classes could be basically MC only like Worlds Without Number.
8) Good rules/guidelines for improved stuff in combat. It's so profoundly odd that 4E, the most regimented D&D game in combat is the only one which had these. If you're ever forced to make more than one d20 roll to do something in combat, that action should be commensurately stronger if it works.
9) Feats more as a tool to customize your character than to just make them stronger.
Etc.