I recommend you pick something with really simple gameplay mechanics that you can evolve and add features to, so that you can finish the game in the time allotted, but if all goes well you have time to add additional stuff.
A simple beat em up is a great idea, since you can add weapons, new characters, enemies, bosses, etc...
For me, when making a game, art is always may greatest limitation. If you can do your own art, that certainly helps, but if you can't I recommend letting existing art assets be a contributing factor in your decision. If you are looking for awesome 2D art that is community license, I recommend you try
http://www.reinerstilesets.de/ or
http://www.lostgarden.com/search/label/free%20game%20graphics since they have a great stuff that has a common art style so your game looks great. Lost Garden has a lot of cool RTS and Shooter art, while Reiner's has a huge variety of sprites (most are isometric) that you can probably fit into any kind of game.
