Shooting things is always fun, esp. in multiplayer, but as you want to focus on singleplayer games, have a look at the following illustration/ sketch I've made some time ago for the community project:

(gravity manually adjustable or by switches/ ...)
You could create a local multiplayer mode (shared keyboard) where the two players control either sphere or box.

Or create a game (jump'n'run, arena jump'n'run, jump'n'shoot/...) where the focus lies on two players that are connected through a string/ ellastic rope.