Needs gutting for new ride that can actually take a decent number of guests. But the spinning coaster in Dinoland at Animal Kingdom remains the best thing for increasing orthopedists' income streams at WDW.
I'm all for a bigger, better Space Mountain. But to completely replace something with something else on this list: Jungle Cruise. I've been going on it since 1974, and it's not aged too well. But it also is the ride with the most variable experience; no other ride is so dependent on the CM who is your skipper. For years we've had a string of great CM's, a string that ended on our most recent trip, when we got a "meh" CM who just didn't have the delivery down. Plus you miss at least 3/4ths of the ride at night. It also has a massive footprint
However, that's not what I'd replace first. That nod goes to Tom Sawyer Island/Rivers of America. This is also a massive footprint, big enough, like the Jungle Cruise, to even put in a new land there.