Generally speaking when I do RP's based on something and will have OC's my two preferences are to either do a story in that universe, but not directly involved in the source material or go from where the source material left off. You see the problem I have with OC's in a fan story RP is that they tend to overcrowd the source material to the point that the source material gets ignored completely. For such a big universe as the DC or the DCAU even doing a story based on a preestablished universe with a history that has canon characters seems like the better choice since creating OC's, and (from what I gather) most people do make OC's willy-nilly (is that how you spell that?). And personally I find writing undeveloped characters much more fun since you have a base for what kind of person they are, but they're free to grow, and it's also more challenging for me.

Anyway my two cents aside I'd be willing to a rotating cast. If we were to have a small group of at least five players it's possible. The way I'd go about it is instead of picking any character from the canon universe at any given time each player will have his chosen characters that only he/she can play for that specific story. This way it would be less chaotic and allows for a more centralized character development.