Heheheh, now that's ambitious.
Go to Rome by all means maybe as an homage to old school celts like Brennus. Vae Victis time x2 only a millenium and several centuries later depending on when you manage it 
I'm thinking on that Ahriman part, I might put a new flavor in. Mongols coming early is a nono because they would just _wipe_ the eastern part of the map. I want the player to have some chance of stopping them if playing in that area. So mongols in a very distant version when I put them in would probably appear 1240s at the earliest. and the Helleno Chinese maybe at the earliest 50 years later... to give the gloating player who survived the mongols a "OMG NO WTH!" type of reaction when they show up.
I'm thinking on that Ahriman part, I might put a new flavor in. Mongols coming early is a nono because they would just _wipe_ the eastern part of the map. I want the player to have some chance of stopping them if playing in that area. So mongols in a very distant version when I put them in would probably appear 1240s at the earliest. and the Helleno Chinese maybe at the earliest 50 years later... to give the gloating player who survived the mongols a "OMG NO WTH!" type of reaction when they show up.