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