I want to do one that is a weird mix of sci-fi and high fantasy about a pair of planets that rotate around each other as well as around a solitary star. The planets are so close that it is possible to see some detail of one planet's surface from the other planet. My RP will be set after a great cataclysm that supposedly ends civilization on Planet A and destroys the known methods of traveling between them. Those living on Planet B are the more normal people/normal races, who have to watch as death sweeps across the surface of Planet A in their heavens. 

Time goes by and life changes, people learning to get by without the benefit of Planet A, until a great flash can be seen on Planet A's surface and the portals on Planet B open to admit a horror the likes of which no one imagined.

The RP would include characters who were surviving on Planet A all this time, as well as characters native to Planet B who are now struggling to cope with a resurgence of magic.

I am pretty sure I won't ever be able to make this work in an RP but I might write a book.