Well, I think I have some ideas for my game <GASP!>

I am going to have a ball that can roll around, or fly (limited fuel...collectable) using keyboard, and fire at enemies/targers using the mouse cursor.
I am also thinking that I may have an 'elements' theme for each location - earth, fire, water (could include ice), air, neutral central hub location, and then some combination locations like fire/water, earth/air...

Different enemies (in locations) might shoot heat rays (fire location: set fire to player for short time - extra damage), freeze rays (water location: slow player down temporarily)...do you see a pattern here? hahaha