I tried an original RP at the old guild once. It was actually pretty damn successful, but I ended up having to drop it (dissapointing a handful of people and making me feel really bad in the process D: ). BUT, it went pretty well, was an "original" RP...but it took things already familiar with players. With the exception of two races, they were all familiar to any fantasy fan out there. Like everyone else has been saying, as a GM it's MUCH easier to keep up with things when the players already know what to expect, or what said creatures are. You don't have to explain every tiny detail and it's just easier to keep up with!

I love fandoms too, I play both. Nothing wrong with a good fandom, so long as it's well put together. I'm working on putting together a Fallout 3 based RP even as I type this up. Just toss in something new, make it your own, and yeah it's a fandom but it's also your own unique RP :)