To add more to that, you really, really need to decide whether you'll accept OC's or not. While OC's are fine most of the time and I play a bunch of them, it's important to consider how it'll affect the plot. OC's can take a higher focus over canon characters in case you wanted them over...
Well, my tip is generally to create something that can stand on its two legs in terms of story. A problem I've seen with a lot of Multifandom RPs is that a lot of times, the setting can feel a tad generic and end up leading nowhere in terms of a memorable story, and generally, it's only player...