Oh man, i have a ton of ideas for games in my head. But, to avoid boring you to death and/or committing suicide because they probably won't ever become real, i'll just give you one of them.
It's an FPS game, as fast-paced as they come, kinda like Serious Sam, but much more sci-fi. You control a typical protagonist, who might not be human. From checkpoint to checkpoint, you are under a constant and intense assault, giving you very few breathers and keeping you on the edge of your seat all the time.
Enemies have ranged and melee attacks, with the former being either instant hit or a projectile attack. Instant hit is weaker, but undodgable (aim is slightly delayed though), while projectiles can be avoided and/or shot down, but are dangerous if they hit. You almost always under attack, so you'll have to dodge a fusillade of munitions, all while returning fire. From time to time, you'll also have to face deadly bosses, some so powerful that you actually have to run away from them in order to proceed. Naturally, they won't make it easy for you. Keep running, or they will destroy you with a melee attack.
The protagonist himself is a very strong, tough and well-armored individual, so you can take a good deal of punishment when compared to ordinary humans, and you can beat most non-boss enemies in a one-on-one brawl. This is compensated by the great number of enemies, however, which could easily exceed one hundred per checkpoint, which again could mean that you got swarmed by melee foes while being pelted from a distance.
The game would have some sort of story, though nothing particularily epic, considering the nature of the game. There will, of course, be more to do than just going from A to B while killing enemies. You might also have to hold a position, drive a vehicle or even do one of the dreaded escort missions.
I could keep going about that game. :U