Well the biggest thing that seems implausible to me is the language of all the place names - I can't quite figure out what it's supposed to be. The capital of the whole Empire, for example, seems to be in territory which belonged first to the Rasna and then later the Celts. A lot of the names seem to be corrupted versions of Greek, but Megale Hellas never reached that far north - perhaps a synthesis of Hellenic and Celtic culture? Doesn't really explain everything though it would explain why they seem to rule most of both the Celtic and Hellenic worlds.
In general I'm tempted to say an Empire along these lines is possible. Alexandros Megas, for example, ruled essentially the eastern half of the map and more. Of course this empire would seem to after the fall of the various Diadochi, given that the Parthyaea have moved south and overrun most of Iran. Presumably given time they would expand further westward, since this seems to be in the early migration era. Presumably the Saka and other Iranians are pressuring them from the west, and in a few centuries the Germanic peoples are going to come under pressure from the steppe as well. I expect, however, with such a unified empire those peoples would be easily assimilated.
[I'm assuming that the script and date are using some translation convention, since neither those characters nor that specific date would make any sense in a world without a Roman Empire]