I have a text file that I write down all of my ideas into. It's easily over 1000 by now. I've kept it going for years. Here's a small random copypasta from it:
- Spaceship/FPS where you mine/steal stuff from various asteroids/abandoned bases (maybe to upgrade your ship?)
- Mage game: whole game is spell casting instead of melee. (maybe w/ wands? Include wands?)
- Game takes place in caverns: 2x heightmaps ontop of each other
- Game in which you glide between platforms avoiding obstacles to reach some goal.
- Buy+Combine supplies to make stuff to sell in your store.
- Grave robber: dig graves, rob tombs for treasure
- Build a base to survive a constant onslaught of zombies
- Salvage: Need oxigen to survive on broken space stations/ships to collect junk to upgrade your stuff. Start projects at base which take a specific amount of time.
- Ninja/tenchu game: Tile based (free movement? Seek mouse?). Anyone who sees you attacks you, otherwise you just have to assasinate someone. Hide in stuff, behind stuff, etc. Enemy visibility is always shown for whole map.
99% of these ideas never make it past the idea stage.