Answers can be specific (the Whoniverse, Planescape, Middle-earth), or simply a setting style (like steampunk, cyberpunk, galactic exploration). My favorite is probably the Planescape setting for D&D. The possibilities are endless, things don't have to conform to the rules of the universe if their presence is strong enough, and ideas are living things that can become reality if enough people believe in them. Men can become gods and a strong enough will can change the nature of reality and the rules everyone plays by. I'm a fan of chaotic, malleable settings. Also retro-futurism.