If I was all powerful and could do anything I wanted:
Monorails everywhere of course.
Bring back If You Had Wings, that brings back such fond memories.
Alien Encounter, Kill Stitch.
Bring back Horizons.
This is one no one has brought up yet add more thrill rides to MK, the thrill rides aren't that thrilling. Space mountain is fun but it only goes 45MPH not very fast. Big Thunder Mountain I think is slower. Both are fun but not heart racing, not like RNR. Do a theme ride and at the end make it a roller coaster like Splash mountain but the coaster part would be higher, faster, with flips and twists.
Bring back River Country.
Old Figment.
Like someone else said there needs to be a 5th park, I've sat and thought about what the theme would be and can't come up with a single thing. Maybe just a roller coaster/ hight speed park. Not much themeing just thrills. There is a park in Georgia that is growing up it has some pretty good rides but for every adult ride there is a childs ride just like it beside it. At least there was last time I went. That would be a pretty cool idea for a 5th park.