We played a lot of TORG in high school. If you aren't familiar with it, here's the basic premise: earth has been invaded by a bunch of different "realities." Each of these realities is a different genre, so there is generic fantasy reality invading Great Britain, a cyberpunk reality invading Japan, a pulp reality invading Egypt, etc.
Do you ever look wistfully at your shelves and shelves of games and think, "Man, I wish I could run some of these settings I've accumulated over the years?"
Why not run them all at once?
What if earth was invaded by the "realities" of the game settings you've got collecting dust? Here's how I'd go about it: