[@Conejitoooo]
Before that, I think we first need to establish the broader game mechanics it'll operate under. If the players are up for it, we could incorporate some sort of tabletop system. I did some research on the subject a while back, and I think the [url=http://animaprimerpg.com/]Anima Prime RPG[/url] system would work well for forum-based tabletop gaming. D&D is designed more around movement mechanics, which forum games suck at. There would be a somewhat high barrier for entry, in that whichever game system we decide on (if any) would require players to become familiar with it. So I'm hesitant to use any tabletop system at all unless a majority of players are keen on it.

If we're not using tabletop mechanics, we still have a number of other mechanical decisions to make, like what kinds of skills players can get, how they get them, how combat is resolved, and so on.