1. An asteroid, meteor, or comet is on a collison course with the Earth.
...
100. A super-villain assembles an army of villains to conquer the Earth
THATS WHAT I'M TALKIN' ABOUT!!!
*sniff* Methinks I smell a pro lurking about...
Additional:
Heroes switch minds & bodies. To do this, randomly determine who is playing what PC. Everyone uses a different PC, but each PC has the INT characteristics & INT-based skills of their original PCs. You might even require WIS or INT checks to determine if they can properly use the new body's abilities...and they WON'T know how to work some of the various gadgets.
It requires a little work on your part, but it can be goofy fun!