My game idea that I don't have the motivation or skill to start:

2 player co-op. One player pilots a tank with a forward facing gun. The other controls a turret with a big gun, small gun, and grappling hook. Power ups can be grabbed by the hook, as can terrain and enemies. View is top down arcade style. Gameplay is shoot em up style with weapon and tank power ups. Tank would have shields, and an energy system that can be allocated to shields, repair, guns, or engines, in various proportions. Pilot controls this allocation.

Enemies wouldn't use the same formations and paths each run, so you can't learn their patterns. Gameplay could be as fast or slow as you want.

Feel free to steal this idea, or pour scorn upon it. I've had this idea for 35 years now.