By my opinion playing a forum RP is effectively the same as writing a collaborative story. So above everything to be a decent RPer you need to be at least a passable writer.
Why's this important, though?

Well, basically what I am thinking about is a sorts of "circle". About what, though?
Did you ever have a dream match between two characters? Say the ever-infamous Goku vs Superman.
Well, this idea would sorts of serve to satisfy that. You submit your idea and people who signed up for this game would write a one-shot story about these two people (or perhaps more) duking it out.

That's the basic idea. I'm rather short on time now so I end it here. I'll explain the details later.
Anybody's interested?