The Pitch: 2010's Tron: Legacy concluded with Qorra (Olivia Wilde) entering the real world, and director Joseph Kosinski recently confirmed that the third film would be an "invasion" movie, in which Tron's inhabitants leak out into the real world and start a war against humanity.
It's certainly an awesome hook and a unique enough idea to differentiate itself from the previous two movies, one-upping the stakes and allowing some distinctly new visual possibilities for the franchise.
What Happened?: A third film was put on hold after Legacy underperformed at the box office, and as of late Kosinski has deemed the film in "cryogenic freeze", but refuses to state that it's dead or definitely not happening.
There have been rumblings that Ascension's script might be repurposed into a reboot starring Jared Leto, but nothing's concrete yet.