Real time based games are much harder to code than turn-based ones. Roguelikes are probably the easiest, you can create good ones in as little as 2 days (except for assets).
You could also just invent some kind of board game.
If you are capable of making a platformer, though, and since you mentioned tower defense. Why not make a tower defense platformer hybrid game? Like a tower defense game that you see from the side with ladders and stuff. And you can carve statues and stuff into the wall in the back that will attack the units. And when the phase plays out you also take part in it and can jump around and climb ladders and shoot attackers.
Back in high school, we made some 2D side-scroller for our project. Now that I think about it, it's pretty much like the Mario games, but with no platforms.
It was really simple, you were given a 'package' at the beginning of the game, and you had to deliver it to your friends house (at the end of the map). There were monsters in the way, so you'd either have to jump over them to dodge them, or jump on top of them to kill them. Once you make it to the end, you win!
I'd say go with side-scroller/platformer, OR, something with a 2D grid-movement system (like the older Pokemon games).
Short project?
I would do a side scrolling action game.
Maybe a ninja. Basic slash attack. Projectile attacks based on pick ups. Shuriken, Bow, Magics.
Allow wall jumping.
