Random idea. Think about it, what is the smallest action that a game could involve? With the biggest outcome? Imagine if you could "control" an atom, or a tiny "nano-machine", you could make huge changes... Ie. If you were the last "chemical" needed to be combined to create the conditions for life. Or, if you were to set off an explosion by mixing chemicals. Of course, it would not be necessary to actually simulate real chemistry or atomic structure, but a very simplified version could be interesting. Imagine "piloting" an atom around a level, (think of the gameplay in Thrust or Lunar Lander) trying to hit a target, you would have to avoid hitting chemicals "that would bind you into them. Different chemicals could react differently when hit.

MMO-sperm racing, each player tries to be the FIRST to find your way to the egg! No prizes for second place.

Of course, you would have to respect the uncertainty principle, so that you would in fact control a wave function across a sea of other particles wave function. You could never know exactly where your particle would be heading and where it is.

Being an electron, you could be annihilated by a vile positron..

And particles could appear randomly (quantum fluctuations) ..