Oh, I would do so much.
1. I would make Tomorrowland more future-like by adding more neon lights, painting more things white/chrome, etc. I would add more effects to Space Mountain, kind of like the effects at Hong Kong's Space Mountain. I would replace Innoventions with an Aliean Encounter ride, and I would put the PeopleMover back up on its tracks. I would update Honey I Shrunk the Audience with a whole new show themed to the movie (was it a movie?) Tron.
2. In Adventureland, I would do a MAJOR rehaul on Indiana Jones and have all the effects working again. I would maybe add some new effects here and there just to spice it up. The Aladdin restraunt thing would be removed and a themed flat ride would go there, similar to Tomb Raider: Firefall.
3. I would completeley retheme New Orleans Square to be a pirates themed land. Since eveything in New Orleans Square seems to be transforming into pirates-related things, I think this would be a pretty smart descision. The open area behind Pirate's Lair (formerly Tom's Sawyer Island) would be home to a new roller coaster called Fire Mountain. It would be kind of similar to Mystery Mine at Dollywood.
4. Last of all, I would refurbish all the buildings and old rides in Toontown, since they seem to be slowly againg and looking worse and worse every year.
That is all.