You can never finish your damn games! So this is your chance to finish something. Either start a new project and just get it finished, or finish that bugger you've been meaning to for ages! It's time to learn a new tool, or not. Do whatever.

I know that one of the hardest parts in game development is themes, so I'll give a genre and a theme but you by no means have to apply to these. It's just the ideas. 

Underwater. 
Platforming.

Take one of those, take both of those, take neither of those. Doesn't matter as long as you finish a god damn game! If you want more things to tie you down, use entirely open source software (even the OS if you can ;) ) because you know deep down that open source is good for you :P