DHS is on track to get a single mediocre Family Coaster and Epcot should be getting its' Guardians Attraction. While the two Star Wars Attractions and Flight of Passage should be great and more intense, impressive coasters aren't really in abundance. Guardians should be the first notable add on since Everest.
Magic Kingdom has only 4 coasters, and 3 of them are more on the mild side.
There's certainly room for a new addition, whether Tron or something else.
More critical in my eyes is getting Magic Kingdom something boundary pushing in terms of technology and scope. A true 21st Century Attraction. The other three Walt Disney World Parks will all have cutting edge super E Tickets within a couple years, Magic Kingdom deserves to join their (and IOA's) ranks. Every other Castle Park except Paris has one or will have one soon. Tomorrowland presents an interesting opening to use technology in compelling ways. Magic Kingdom feels old, and needs a shot in the arm.
Tron would fit the bill. That attraction is awesome and would instantly become one of the top rides in Orlando. It's the type of attraction teens and young adults would rave about. So would something along the lines of Battle Escape or Battle for the Sunken Treasure.
They could also modernize Space Mountain
Though I'll once again add my disapproaval for removing the Speedway before other expansion pads have been utilized. Seems like another way to save Disney money and mitigate capacity improvements. Let's add capacity, not trade it.
Magic Kingdom is perhaps Disney's most important Brand, powering billions in revenue and profit. It's time they showered it with the attention it deserves.