The game I had in mind was something similar to the arcade game "Metal Slug."
If you are unfamiliar with Metal Slug, it is a sidescroller where you can shoot various types of weapons, for example a pistol, machine gun, or rocket launcher, and try to get to the end of a level and defeat the boss. You get a certain amount of lives and the weapons you pick up as powerups with a set amount of ammo.
My game will have the same basic mechanics, go through a few levels while being to able to upgrade weapons and defeat enemies and a boss. I would love to be a able to do co-op, but not a necessity. Another aspect I want to eventually do (not necessarily in the time frame of the course but possibly after) is an RPG element. Where you can level up and possibly choose different powers, like extra health or damage for a small amount of time on a cool down, or have the ability to level up a weapon, after you kill x amount of enemies with the pistol you can increase the capacity of your clips, or something similar. Currently I want to implement a single level but I do want to make it extensible, so I can continue to add levels, just a matter of coming up with the content.