What I think I would have done, or somewhat expected...
- Move back from 4th edition toward a more 3ed game design.
- Two progression tracks for pretty much everything (BAB, skills, saves): good (2+ 1/2 level) and poor (1/3 level) nothing in between. Proficiency basically gives you good progression. Changes are retroactive when new proficiencies are acquired.
- Three or four tiers of play, each supported with capstone abilities in classes, advice to E5, E10, E15 or E20 games, magic items and monsters divided in tiers.
- Vertical and horizontal class symmetry. All classes follow the same pattern: first level = apprentice level, second level first meaningful level with a +1 bonus/5 level (bard would be bonus to knowledge, ranger to enemies, fighter to damage, paladin to AC etc), third level offers a bunch of options etc, on a 5 level rotation for four tiers of play.
- Less classes but more internal options to make fighter more like a barbarian than a knight, more like a paladin than a cleric, more like a ranger than a druid etc. Same with races. [enter inevitable option bloat with splat books].
- Some sort of upcasting or scaling spells. All bigsby's hands spells folded into one scaling spell for example.
- Rationalization of bonuses/penalties. I prefer advantage/disadvantage, but I honestly didn't think of that or expected something like that.
- "Weponization" of wands, staves and rods with +1 to +5 bonus (spell attack bonus for wands, DC for staves, upcast for rods, etc).
- Some kind of regular vs flat-footed AC formula. I must say I'm not missing it however.
- Max of 2 attacks per round (perhaps three with two-weapon fighting), all with same bonus (no itterative)
- A feat system. Probably with some kind of controversial feat chains or feet trees.
- No or slow stat increases
- Abstraction of "healing" (moral, courage, second wind, divine touch etc.) to be consistent with abstract nature of hp.
- Some sort of "natural 1 is always a fail because a check shouldn't be called if no chances of failure" philosophy.
- More build-in minigames with crafting items, wilderness survival, downtime etc.
- Return to a vancian casting system of some sort, because nostalgia/sacred cow. If I had to guess, one much more class-specialized and contrived than that of 5e.
- Reflexes/fortitude/willpower saves.
- An honest but ultimately failed attempt at reducing reliance on the "big 6"