I wasn't going to reply because, well, I haven't made any RPs in a while and none here. But you've gone unanswered for a while, so here is my input. It is probably best to make your own; I've not seen RPs for either of those series. Before starting, I suggest reading this thread:
http://www.gaiaonline.com/guilds/viewtopic.php?t=9607055This thread describes the different levels of RP in this guild. While all new RPs start in the main forum, determining what you want for the future of your game may help you determine how you want to set up your game.
My own personal tips?
Think about setting. While you already know it, imagine you are presenting this idea to players who have never been introduced to it before. What do they need to know? What are the important events, people and places? What history might affect their characters?
Think about characters. What kind of characters are you looking for? This is closely tied to setting. You don't want Picard running around on Genosha any more than you want Wolverine slicing up the USS Enterprise. Are you going to allow characters that exist in the series? Will you allow original characters? Let your players know what kind of characters they are allowed to play and give them a profile skeleton to fill out. How detailed this skeleton should be is really up to you.
Think about combat. Will there be combat? Will that combat have rules? Are there only certain accepted powers or weapons? What enemies might the players encounter? Will players be able to play those enemies? Anything like this, your players need to know.
Basically, all of my suggestions boil down to detail, detail, detail. Don''t just make a single post that says "hey, this is an X-Men RP, you can play anybody but Storm cuz she's mine." Introduce your RP to draw in the players, describe it to answer the questions they didn't even know they had yet. And by all means, look at some of the already existing RPs here. See what common factors keep those long running ones going.
These aren't strict rules or anything like that, just my opinions. Feel free to put your own personal stamp on your RP; that's a good way to stay interested in running it.