My idea
I plan to make a tower defence/exploration game. You don’t have to pay anything to build a tower, but when you fire you use up ammunition. You get new ammunition in the exploration stage.
I’ve ‘Kinda’ got an idea for how to tie this in with the theme–the ammunition you collect is poison gas that you harvest from mushrooms growing in a swamp. Maybe I’ll think of a better idea later.
Good luck everyone!