As cool as Tron looks (I've never gotten to ride it, my last trip was right before it opened, and my next visit is early June this year so I likely won't get to ride it lol), it didn't do much to help out Tomorrowland. They really need to do something with the Alien Encounter/Stitch theater, and I think Monsters Inc Laugh Floor needs to be replaced. The footprint of it is small so it'd probably have to be another theater show, but there's also areas elsewhere you could expand.
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Of course there's the former Stitch theater that's empty, and the Speedway which is garbage and should just be replaced with something else (I've suggested Sugar Rush, since from Fantasyland, a candy land would visually fit the fantasy theme while from Tomorrowland, you'd be entering a digital world). You could fit like, two rides on its footprint if you wanted- either two for Tomorrowland, or one for Fantasyland and one for Tomorrowland. If you did a few Fantasyland dark ride-sized attractions, you could even milk that into three attractions!
If you were to do something other than a theater show with Laugh Floor, you could take the space of Tomorrowland Terrace and make a small-ish dark ride or something there. No sure how sacrilege that is, given I've never heard anyone talk about that restaurant and I don't even know what it does differently compared to Cosmic Ray's aside from outdoor seating.
You could hypothetically build something to the right of Space Mountain with some land preparation, maybe using the bathroom there as the footprint for an entrance and have the queue go under the train track like Space Mountain's entrance. A dark ride perhaps? There's already two thrill rides and a shooter, so something without a height requirement would be a good addition. WALL-E, perhaps? Dare I say it, but possibly even a new Horizons, since it'd be right next to the Carousel of Progress? Fantastic Four maybe, given the new movie's art direction is supposedly inspired by the Tomorrowland of the 60s? Then to deal with the bathroom removal, you could move it between Buzz and CoP, not needing that entire plot of the parking lot I highlighted.
Plenty of space to expand!