A new tomorrowland coaster. I know that sounds weird because you already have space mountain. However, tomorrowland is somewhat lacking beyond space mountain, and that ride itself has a very long wait. Tland is already attracting the coaster crowd so why not?
I say demolish the astro-orbiter area. Move the food over to the old skyway station. Rebuild the center area with a cooler taller astro-orbiter that has a much better line. Now, with a bigger center, make a people-mover coaster.
Disneyland had that one weird ride for a while, but I say make it a legit coaster.
You would have to do a lot of reconstruction in the two tomorrowland buildings, but then you could do a substantial retheme.
The only question people would ask is: why two dark coasters? And the answer is: because it works and the capacity is there.
Why have a carosel and dumbo and teacups? Each is fun and a little different from others.