WI: Rome and Persia make love, not war.
We've discussed Roman Mesopotamia, and (less so) a Syrian Levant which oddly is grimdark and dystopic in a way the former isn't. But what about a third option? Caracella gets a bum rap, but one of his more interesting proposals was to marry the daughter of Artabanus V of Parthia. As Herodian put it:
"The two most powerful empires were those of the Romans and the Parthians. If they were united by marriage, [they] would create one invincible power no longer separated by a river... Furthermore, the locally grown spices of the Parthians and their wonderful clothes, and on the other side, the metals produced by the Romans and their admirable manufactured goods would no longer be difficult to get and in short supply, smuggled in by merchants. Instead, both sides would have commerce and unimpeded advantage from the unification of their countries under a single rule."
And of course the Parthians were better neighbors for Rome than the Sassanids would end up being. Even so, the two states could and did cooperate on occassion, notably during the 5th Century. (And in an alternate universe against the space nazis from beyond time).
So, what about the idea of Persian-Roman coexistence?