MK: better/more sit down (or even just nicer quick service) dining options. I literally plan our trips around not having to eat big meals at MK. I despise the craziness around getting a table at BoG, CRT is so expensive for what it is, and the other options are, in our experience, just...OK. I can think of many, many other places at WDW I'd rather eat than pretty much all of MK right now.
Epcot: ...oh. OK, if I'm picking one thing, let's say re-opening one of its closed buildings in Future World for something other than event space.
Studios: does it have to be a realistic change? I would love to see the working-studio-aspect back, but I think that ship has sailed. Honestly I'm having a hard time with this one. The actual changes coming should be a nice start, but something's just off about the whole vibe there over the past 5-10 years now. I'm not sure what will "fix" it.
AK: another family-friendly (but not necessarily "very little kid-friendly") ride or two