Imperials are the most straightforward; I don't get anything but pure Roman Empire from them, even moreso in Skyrim.
I dunno. Cannot think of any empires that rose and fell multiple times in Europe. China though...every friggin few hundred years. Splintering, Invasion, rebellion, re-conquest, new dynasty, stability, repeat.
Really besides the names, military structure, it being an Empire with a "senate"(Elder Council) and Roman armor there really is not all that much that fits as a Roman thing. Hell they even have a whole mandate of heaven thing going with the Amulet of Kings. The Mandate is all chinese. Dynasties are very important for China. Rome...not so much. Important but not incredibly so.
I think it is more a combo of those two plus many somewhat original ideals.
I cannot think of any cultures that have risen from slavery against evil masters and created an incredible empire that conquers an entire continent multiple times though. There are many things that make them quite unique. Ancestor moths, Elder Scrolls, White-gold tower, Niben Bay, a jungle empire turned to grasslands by a Demi-god turned into a god after death(Talos). There is loads more.