At this time it's hard to tell, cuz I'm not even sure which edition of D&D I prefer anymore, and I tend to prefer skill-based over classes anyways. But they all have things I like and things I absolutely HATE, and I'm not sure which I'd pick and choose since they all play very differently and are essentially different games. I'm not even sure what's "D&D" is anymore!
5e feels completely different from 3e, which was significantly different from 2e, which are the editions I've played the most. And 4e was something else entirely. B/X was also very different as well, but not so different from AD&D than from what came later.
I would keep the d20+Modifier mechanic, though--I'll even use it in non-D&D games--that's the one thing I can tell you with stone cold certainty. And I would make
everything a skill (the way it should be!) if it where up to me, as well. Attacks, saves, activities you can do--
EVERYTHING would be a skill. Much like 5e already handles it now.
I also like the way that 5e handles skill progression by automatically advancing based on your level. Though, individual skill progression (3e style) can be good as well, depending on the system and the type of feel you're aiming for, but it can also be tedious as hell. And most people are gonna max everything out anyways.
But I don't like the way that in 5e you either have Proficiency or you don't, with nothing in between. And expertise just doubles your modifier, which is too simplistic and can produce too much difference between the haves and have-nots. I prefer the way that PF 2e handles it with multiple proficiency ranks: Untrained, Trained, Expert, Master and Legendary. And each rank above Untrained basically gives you an incremental +2 bonus. I don't like the way that PF 2e adds the
entire character's level to as a base proficiency modifier, though, and would probably just make it half level (since 5e modifier is kinda low as is, but full level just adds to power creep and ridiculous modifiers). I would also go with old D&D terminology for the proficiency ranks: Skilled, Expert, Master, and Grandmaster.
One thing I DO NOT like about editions 3e onwards is how character classes tend to be a bloat of features, and how tiny conditional benefits that are situational and don't do much have become the norm. I DO NOT like keep track of dozens of minor abilities that do not add anything substantial to the character's capabilities and just add to a bunch of bookkeeping. I hate bookkeeping and it just detracts from the game. Sometimes it might be necessary but we shouldn't pile on it with gimmicky class features that exist mostly to pad class levels and kinda sorta match them up to casters and more solid classes without giving them too much.
I would also like to break all class features down into Feats, and just handle every special benefit or ability as a type of feat (focusing primarily on stuff that's substantial and worthwhile, rather than gimmicky minor stuff).
Even spell casting should be broken down into feats to access Spell Levels and Spell Lists (or equivalent power lists, like Psionics). Assuming I keep the current 9 level progression (which I don't like, but fixing would be a separate issue), levels 0-3 would be one feat, 4-6 a second feat, and 7-9 a third feat. Spell Lists would be divided into Minor (Paladin, Ranger) and Major (Mage, Priest) lists, with major lists further broken down into schools or equivalent (psionics disciplines, etc.). Minor spell lists might be one feat, and major lists would be broken into two-school selections per feat (school equivalents that offer broader access that schools would count as two school selections).
So my ideal edition of D&D right now would probably have something like:
- d20+Mod action resolution
- Everything is a "Skill"
- All special benefits including traditional "class" features is are "Feats".
- Proficiency modifier is +1 per 2nd level, +2 per rank above Untrained (0 for untrained characters)
- Proficiency ranks are: Untrained, Skilled (+2), Expert (+4), Master (+6), and Grandmaster (+8).
- Less bloat and bookkeeping.