I second RWBY, but I have played a game where all fandoms could play. The character just showed up to the setting, and they don't know why or how. (Rifts in space/time were randomly showing up, but that was OOC info). The biggest problem with it was people who wanted to play OP characters.
Also, I think this should be all OC's to prevent the "But I want to be (Insert character's name)!