A game idea I had, but probably won't make, is based on the old style bullet hell shoot em ups. Typically, you flew an aircraft or spaceship over a landscape, shooting oncoming enemies and ground structures, while dodging a shitload of incoming shots and missiles.
Now, here's the twist: Make it turn based.
So, instead of a single ship, you have a large, tough ship (with turrets and a big cannon) you need to escort to the boss (you need the large ship to destroy the boss) using, say, two fighters and two bombers. You give your orders, everything moves a little bit, then new orders. You'd get the same "ballet around the bullets", but turn based.
So, here's the next twist:
The boss represents another player, and each player builds their own defenses, placing turrets, ships, powerups and so on. Now, how do you balance it? Well, before a player can be attacked, he first have to beat his own defenses, to show that it can be done.
Then, you score points for successfully attacking, and for successfully defending.
If you make it, tell me, because I'd love to play it.