The Universe Is Toast
The Universe Is Toast (often abbreviated as TUIT) refers to the title of the expected, but unproduced trilogy following Keen 6. In it, Keen was supposed to prevent the destruction of the entire universe, taking on his old arch-nemesis Mortimer McMire. Sadly, id Software decided that they had more productive projects to work on and the series remains merely a suggestion to this day.
There remains some support for the series, however, mainly in the form of two of the original id team, John Romero and Tom Hall. Tom especially has often expressed his desire to personally contribute to TUIT, most likely as a 3D platformer game. As yet, the only remaining obstacle to its production is the refusal of id Software to release the intellectual property rights back to its creator.
In Tom's own words:
"Well, I don't want to talk too much about it, as I may do it someday, but as you know from the end of Keen 6, Mortimer McMire is back, and he has big, bad plans for the end of the Universe as we know it...
If I can ever get the rights back to Keen, or if I can strike a deal with id so I have creative control of him forever, then you'll see Keen again. I don't want to start Keen back up, only to have him taken away again if he's successful. That just wouldn't seem fair.
I would love to do another Keen. My last idea for Keen 7-9 was a game world [that] was 3D, and at certain places the camera rotated with you for different games. It was halfway between Super Mario 64 and Pandemonium. This was two years before those games came out. I wish I'd been in a place where I could've made that happen back then.
I do miss the good ol' days of Keen. I love the universe and the gameplay, and I'd love to make another chapter in the saga. We will see how the future unfolds.." 
Joe Siegler recalls what else Tom Hall said about Keen 7...
"When Tom Hall was working here at Apogee, I used to pester him about Keen, because quite frankly, I was a customer of Apogee's before I started working here. I used to ask him a lot about what he had planned for Keen 7, and I remember him being really annoyed when Super Mario 64 came out, because Tom said that was exactly what he had planned for Keen 7 - he wanted to be first to market with that kind of game concept.
When I asked which characters he said would be back from the original Keen games, he replied "All of them - a level Idea I had was that in the first level, you'd have every single character from all of the previous Keen games in there all at once - sort of a Galactic Zoo type of thing. Then I'd start introducing new stuff after that." Whether or not Tom actually uses this idea or not, you can tell he was thinking on a grand scale." 
- Romero's Keen 7 Tech Demo
- Episode X, Episode Y, Episode Z, three mods based on this series created by Xky Rauh, Grelphy, and Ceilick respectively
- The Universe is Toast (mod trilogy) - three mods based on this series created by Ceilick
- TUIT (fangame) - a fangame based on this series created by Fleexy
- A Look Back on Commander Keen - source of the above quote and the name "The Universe is Toast"
- Romero talks about Keen 7 prototype prior to release