With a POD starting in 2nd century BC, have the Roman Republic not become an empire until about the 2nd century AD. Then, have this later Roman Empire by the 6th century be more powerful than the OTL Roman Empire and control the entire middle-east. Then in the 9th century AD have the empire split into two parts as in OTL, but have the Eastern Empire collapse first in the 10th century AD and have the Western Empire survive until about 1500.