If what you're looking for is just roman-themed nations in RPG settings, don't overlook the Empire of Thyatis in the original D&D known world setting, Mystara.
Dawn of the Emperors is the box set that details Thyatis, and its arch-nemesis Alphatia.
Also in the Mystara Hollow World setting, there's the Milenian Empire, which is really more greek than roman, but has that same classical empire feel. That setting also has the Nithians, who are blatantly Egyptian.
On the other hand, if you're looking for a setting that has a single "world-wide" (or continent-wide) empire that captures the "sole power" feel of Rome when it was at its peak, I don't think I can help you with that.
I think that would make for a very interesting setting; sadly, I haven't been able to see Morningstar.. is it like that?
Nisarg