Here are some ideas I have done before:
-Set in Post-Apocalyptic New York, a series of people survive in the new era and it was basically a roleplay about living in the aftermath of WW3.
- Alternate reality where Magic is prevalent in the world, but went into hiding during the witch hunts. The main characters were hunted by the ruling council based in Europe because they belonged to a clan that was wiped out due to their betraying the Council.
Ones I haven't done, but they might be cool:
- Steam-punk styled fantasy world where magic has replaced most modern technological advances so that they are on par with about mid 1800's technology. Two major countries on the center most continent are on the verge of going to war over a small patch of land, until it is destroyed by an unknown assailant, forcing the two major countries to believe the other had started a war. Characters would be from the smaller country as survivors.
- Alternate Earth, where magic existed, and helped avoid the Dark Ages. The Renaissance Era came right after Rome fell, and Technology changed accordingly, so that the setting of World War 2 would be fought at magically influenced future technologies that would date as 50 years ahead of present day reality. World War 2 will have been going on for nearly 100 years, where 18 years ago, in 1991, England finally fell to the German forces, and America was fighting on its own home front, as well as staging from Japan into Italian controlled China, and German controlled Western Russia. Russian survivors are staged in Siberia and camped in Alaska as well.