Tron could fit if they tore everything but Space Mountain down, built a multi-level facility, built the canopy over the lagoon, removed Autopia, or did whatever combination of the previous options is necessary. That would be cool.
Regardless, Tron is one of the few sci-fi oriented franchises of merit Disney really has. And it's mainly due to the influence and permeance of the Tron aesthetic rather than the quality of the films themselves. Other than Tron, you have Big Hero 6, WALLE, Avatar, and one day Lightyear. You could maybe swing the Incredibles if you go down the retro futurism path, but they already have a presence in DCA and I doubt that would go over well. Because of this, I imagine Tron inevitably receives a big presence in Tomorrowland, albeit with a different ride system. Flight of Passage also would make sense (you already 'link' into a body that's far away), but only if they don't theme the showbuilding lazily. Big Hero 6 could be a nice edutainment dark ride. Walle...is too depressing.