For all I've said about wanting Villains Land at MK, I do really think that Disney needs to focus on the 3 Non-MK parks first. It seems clear that isn't quite what they have up their sleeves, but Magic Kingdom is largely a victim of its own success. If any of the other 3 parks (ideally ALL of them) were properly engineered to pull their own weight then MK wouldn't have to scrape for every ounce of capacity it can manage.
Of course it's reasonable to want MK to get some nice new toys from time to time as well, but I think the push to expand out behind the Rivers of America comes largely from Disney wanting to bet on its fastest horse in the face of Epic Universe. They see MK as the park that pulls the guests in and EPCOT, DHS, and DAK as the places guests go when they're not at MK. A big new expansion to their headlining park serves the double purpose of pulling the largest potential group of people to the resort as well as theoretically helping solve the capacity issues they already have at that park. But I think if you keep doing that Magic Kingdom just continues to stand head and shoulders above the other parks in terms of offerings, and the prophecy fulfills itself.
This to say, I think thematically I'd prefer a Villains Land at MK, but I wouldn't say no to one at the Studios. I definitely think DHS needs much more than it's gotten, and real, genuine, jaw-dropping expansion should be a high priority for that park.