Not very complicated and story-related like other people's idea, but a much simpler one that would give many people tons of fun.
So, you're a (color of your choice) cube running through a selected tunnel, with the primary objective to not die, and to get to the end before your time is up.
There are various things along the way that may help you, powerups, such as a speed boost, temporary invulnerability, an extra life, and many others.
There are also hazards, such as spikes, which may come out of the ground or through the ceiling, maybe even launching itself when it has been triggered by the player. Icy surfaces, slow jelly, and a spring that hits you back a good distance are other examples of what could come up, as well as obstacles that disguise as real powerups to damage or kill you.
There are different modes such as normal, where you get to the end before time runs out, or survivor, where you survive for as long as you can. You can play these in Single Player, or Multiplayer, which is where the fun really is.
At the main menu, you can either click "Play", or the "Be a friend" button (name pending), where you can find a game somebody else is playing, and become an obstacle controller, where you randomly throw in obstacles to try to screw them over. However this would only work with a good amount of people, which is why I'm worried on whether or not this would even work. With the right amount of people, though, it could be VERY epic.
I would make it, but I obviously have IsoLand to work on at the moment so I wouldn't have much progress on it. Still, I think this is a pretty interesting idea and I loved sharing it with you guys.
