I just thought of something which would change the history of the old world quite a lot. What if the Seleucid Empire lasted significantly longer and kept its empire? The implications of that would be enormous especially for Rome. The empire could have possibly spread beyond its historical boundaries to Greece and Egypt. There would have been no mithridatic wars, Rome would have had significantly different culture, religion had they not conquered formerly Seleucid territories. Two of the more significant Roman gods, namely Cybele and Mithras came from these areas. Not to mention that the wealthiest Roman territories would later be in this region. There would be no Byzantine Empire, and here's something else, there would have been no Christianity.