How about the Saurian Empire of lizard-roman raiders? Or would that be the raiding empire of roman-saurian lizards? Or maybe even Moriarty of Mongo? :)

Whatever, I still think that we need one single larger-than-life nemesis. The leader of a gang, saurians or raiders, may just not be enough. It certainly is important for such a nemesis to have a power base (=followers), but I would still like to see a character that is really exceptional.