Well the first idea that comes to mind is a Match-three Tower Defense game, but I don't think I could make that in 48 hours. If you can and do then let me know so I can try your game!
My second idea is a text-based first person shooter, but that doesn't sound very fun.
I liked the turn-based racing idea that was suggested along with this theme but I'm not sure how to do it. XD
Another one that comes to mind is a match three roguelike. But I've made a Match Three game before and it took me months. I don't think I can do it in two days. XD
Or, CAN I!?
I don't know. It's weird I'm caught on Match 3. It's not really incompatable with anything. We've got Match 3 and Pirates and Match 3 and Platformer games to show that.
A friend just suggested Visual Novel Tower Defense and I'm seriously considering it, though that would mean I'd have to create a bunch of busts for chatting at eachother...
Another friend suggested Bullet Hell Text Adventure. I hope taking theme suggestions from friends doesn't count as collaboration. LOL
Other ideas from friends:
Historical simulation 2D platformer. "The Oregon Trail of Floating Platforms."
Scorched Earth-style tank game X survival horror
Solitaire: The Walking Simulator
Rhythm Pinball
Typing Tutor Text Adventure. Backspace is disabled, and any typos you make become mutant objects that will oppose you.
Typing Tutor Flappy Bird
Find-Object-In-Picture + Match 3 of those objects
Where's Waldo in Rhythm Heaven
Tower Defense Platformer
Rampart Bejeweled
Casual match-3 FNAF clone
SimEarth the TCG
JRPG Goat Simulator
Roguelike Solitaire
Golf Kart Racing
Metroidvania formula 1 racing.
Katamari Arkanoid
Asteroids mixed with pool
Prince of Persia Peggle
Okay I'm seriously SERIOUSLY thinking about Visual Novel Tower Defense. I like tower defense games and I think Visual Novel would be easy enough to add in but weird enough to not fit right. XD