You can't patent or copyright a 'rules system', the only thing you can do is copyright the specific rulebook you wrote.
Which means you can create a game identical to RISK, but use different terminology and put the rules in your own words. You can't use the name itself, and obviously you wouldn't be using their board (which would count as 'intellectual properties').
Either way, I'm drooling over the idea of a Golarion/Avistan Atlas.
I'd love an Atlas to come out, but wouldn't having one to soon have an adverse effect on some their future projects?