Most people here already know this, but in case you don't, 116-117 was the territorial height of the Roman Empire. It was also the only time in history when the entirety of Mesopotamia was formally under Roman control. Emperor Trajan, after beating the Parthians in war, captured all of Mesopotamia and turned it into a Roman province. However, Trajan died not long after, and Hadrian, his successor, decided to withdraw from Mesopotamia instead, relinquishing the region back to the Parthians. The Romans would never hold all of Mesopotamia again.
So what happens if the Romans decide to continue to try to hold Mesopotamia? For me, it doesn't necessarily have to be during Trajan's war; any successful attempt by the Romans to capture and hold all of Mesopotamia would work. How does this change the power dynamic between Rome and Parthia, as well as history at large?
Please note this though: I have seen similar conversations about this topic on this site before, and it always seemed to evolve into a conversation on whether the Romans should have tried to keep Mesopotamia or if they should have abandoned it. This is not the conversation I want here; I just want a conversation on what would have happened if the Romans did try to keep it.