If we make two assumptions, namely:
- A retro-clone of 4e could be legally made without losing much of the system in the process.
- This retro-clone was meant to be a single, unified ruleset
Here's what my retro-clone of 4e would look like.
For classes, I think I would present a blend of the AEDU and Essentials design philosophy. Namely, martial classes get stances in place of at-wills, and only the "spellcaster" classes get to pick multiple different encounters (others just get several uses of one). I'd include a feat to allow people playing the non-"spellcaster" classes to swap one use of their baked-in encounter power for some other power. I'm inclined to give dailies back to the martial classes... not sure what would need rejiggering to account for that.
My cleric would look different. Since the warlord ate the, "armored melee healer" role and the invoker ate the, "divine will of the gods" role, I think I'd end up merging the current cleric and runepriest and tie the cleric a lot more to the domains for the gods so they could manifest a domain aspect and alter their powers a little like the runepriest and its rune states.
Feats would be downplayed a smidge. There wouldn't be any generic attack/damage bonus feats. Feats would still alter combat abilities, but more along the lines of, "You can shift instead of making an opportunity attack," than, "+1 damage with acid and fire powers."
Inherent bonuses would be standard to get the game off of the weapon/armor/neck slot treadmill. Most of the magic items a PC finds would be consumables, wondrous items, and the other "slot" items, though magic swords and armor would still exist (they'd just provide the property/power, not the enhancement bonus).
Obviously the more oddball, D&D-specific races wouldn't be reproduced, but I could see the racial array looking something like human, half-elf, elf, eladrin, dwarf, gnome, halfling, goliath, dragonborn, goblin, hobgoblin, half-orc, and kobold. I'd like to work up racial utilities for all the races, similar to whay the vryloka and shade have access to in HoS, and offer them as options for people who want to make their PC a little "more elven" than the baseline rules. I can envision a couple of heritage feats for half-elves, one that lets them use their normal racial or elven accuracy once per encounter, the other that lets them use their normal racial or fey step once per encounter and they qualify as an eladrin for feats instead of a half-elf. (That tackles both the, "What if my half-elf is really half-eladrin?" and the, "Most of the feats I qualify for need elven accuracy" issues.)