I've not done a lot of looking into RPG lately... something about the lack of gamers in my area I've felt I could game with. You could package it to work with the D20 system, much of that has been made 'open source' or you could go more generic and let people adapt to their own systems. Either way, concentrate first on the source material... the world, who populates it, the social dynamics, politics, etc. Worry about the system last.