Too many to just choose one:
(1) World Showcase- I would add relevant, culturally-based (not movie-based) rides in the world showcase (e.g., Speed train ride for China's bullet train between China and Germany, Gondala type e-ticket ride between Italy and Germany, and French alps based log flume ride between France and Morroco) and update Soarin to "Soarin over the World" - we're in Epcot, not sure why this wasn't the plan from the beginning??
(2) Future World - I would partner with apple, google, or some other tech giant and re-do future world (including reverse updating Figment and putting the COP where the World of Energy is) so that Epcot is complete and vibrant again
(3) DHS- I would add Carsland, Star Wars land, a good water ride, and a permanent Star Wars-themed character meal and Pixar-themed character meal.
(4) Frontier Land - I would bring back the Golden Horseshoe show/lunch, and move Toy Story Midway mania (since it's a shooting game based off a cowboy) over into this area somewhere (TBD)
(5) Tomorrowland - I would move COP to Future world, I would add a Wall-E feature that roams, I would take out SGE and put in a Meet the Robinsons based flight simulator ride, I would add a "Space themed" (i.e., gender neutral) character meal where COP was with maybe Buzz, Stitch, Future Chip and Dale, and bring back the other future Fab 5)