If I was in charge 4th Edition would be a simple core system, based on the d20 idea but strongly class-based with abstract combat, and mostly compatible with previous editions of D&D. Something along the lines of C&C, but with skills, feats, and tactical-style combat available in the core book as optional add ons. I would not hard-code much fluff into the rules, but include optional rules to make the basic flavor more sword & sorcery, high fantasy, or anime-like as desired. I would simplify monsters back to something like what the stat-blocks looked like in Rules Cyclopedia D&D. The most important goals for me would be keeping the basic game simple, but capable of crunchy development, and also trying hard to make it all fit in a single rulebook.
Some rules ideas....
I would have 7 basic PC races:
Elves: The Charismatic race (+2 Chr, -1 Con, -1 Wis[they're impulsive])
Gnomes: The Intelligent Race (+2 Int, -2 Str)
Dwarves: The Strong Race (+2 Str, -2 Chr)
Halflings: The Dexterous Race (+2 Dex, -2 Str)
Reptilians or Serpent People: The Wise Race (+2 Wis, suffer extra damage from cold attacks), worship dragons, able to look like other races.
Woodwoses (Hairy Wild Men, like Sasquatches, Chewbacca, or Enkidu): The Sturdy Race. (+2 Con, -1 Int, -1 Chr).
Humans: No bonuses or penalties.
6 Basic Classes (which are for adventurers - ordinary people do not have PC classes):
Warrior: Based on Strength. Dwarves make the best Warriors. Only class with multiple attacks.
Wizards: Based on Intelligence. Gnomes make the best Wizards.
Fighting Clerics: Based on Wisdom. Serpent People make the best Fighting Clerics.
Thieves: Based on Dexterity. Halflings make the best Thieves.
Barbarians: Based on Constitution. Woodwoses make the best Barbarians.
Bards: Based on Charisma. Elves make the best Bards.
Humans are equally good at any class.
Other races (1/2 orcs and 1/2 elves) and classes (knights, martial artists, warrior-wizards, etc.) are optional. All class and ability names are as simple and self-explanatory as possible.
And shields would finally be able to fully block attacks, just like in real medieval combat!