My number one want of 5.1, 5.5, or 6.0 is that it's a long time in coming.
Otherwise, the list is a bunch of minor stuff:
- No drow in the PHB (removing them everywhere would be fine, too).
- Pull gnomes into the standard races and put halflings in optional.
- Make tieflings look mostly human, again.
- Get rid of the Barbarian class entirely.
- Put the latest UA Ranger (or something similar) into the PHB, in place of what's there.
- Tweak Sorcerer a bit. Not sure how, but it just doesn't "click" for me. I think there's a disconnect between "magic is in my blood" and "I have to use the same arcane formulae as Wizards".
- Replace the Wild Magic bloodline with a Wild Talent that captures the Psionic feel of doing magic with force of will/personality. (Don't call it psionics, though.)
- Stop referring to skills in the format of "dexterity (acrobatics) check". It's just an acrobatics check. At most, it's "acrobatics (dexterity)", but skills should not be a subset of ability checks.
- Do something that makes tool proficiency more clear. By this, I mean give an actual list and give someplace to check off on the character sheets.
- Make determining surprise a bit more clear. In fairness, most editions have kinda sucked at this.
- Raise the AC on monsters a point or two. Bounded math is good, but this variable isn't quite right.
- Provide a slightly better price list for magic items. I don't want a magic Walmart, but the ranges are
too broad/vague.
- Organize magic items by category. I've never had a time where I was trying to decide between putting a
flame tongue or
folding boat into an adventure, but I have wondered about whether it should be a
flame tongue or
dragon slayer. That sort of grouping would be much more user-friendly.
- Make about as many references to the Forgotten Realms as the current edition has to Greyhawk. This includes support across all books, adventures, etc. Even better, mothball the Realms entirely. Yeah, I know. Not happening.
Of all those, the magic item organization is probably the one that bugs me most. I seriously can't see how that made it to print without
someone saying, "Guys, this sucks to use." At least include tables/lists by type.
I'm not saying that 5e is doing bad. It's quite good actually. But look at the trends:
3rd ed year 2000.
3.5e y 2003.
4E y 2008.
4E essentials(essentially 4.5E)

y 2010.
5th ed y 2014.
I could get behind a 2020 release, give or take. I'd actually prefer the 10ish years that 1E AD&D had, but whatever.