My idea can be summed up in two words "Space Treadmill"
The player is on a treadmill in space (I don't have a reason for this :shrug:), and their goal is to get back to earth before running out of Oxygen. The player must run on the treadmill to generate power, but this depletes O2 quicker than usual. I'm thinking a sort of keyboard rhythm game, although I still haven't solidified how I want to have the player "run". So, while I've been thinking about what direction I want to take this game, I worked on creating a little prototype for the treadmill in Blender. 
I'm going to get as much done as possible tonight, as I may be unable to work tomorrow. I'm in a high school marching band, and I signed up to work on building props tomorrow not knowing that Ludum Dare was this weekend. It looks like it might get rained out and rescheduled, but I'm not going to get my hopes up just yet.
I can't wait to see what other people's ideas are for Ludum Dare 42, and I look forward to playing y'all's games when I get the chance :smile: