I think the usual cliché of "Rome Never Stops Expanding until they have conquered Creation" is a pretty big one that was already mentioned here in many ways, the empire was already badly overextended as Roman troops needed to garrison borders from the Rhine to the Caucasus to Syria to Arabia to Brittania and the Northern African deserts, adding places like Mesopotamia or Ireland or as is more commonly see Germany is just giving the Romans yet another amount of decades of spending resources sending colonists, putting down rebellions, fighting off raiders, building forts and the typical politicking that comes with these types of major decisions. "Just one more province bro, I promise you bro, this is the one province that's gonna solve all of our issues bro I promise"
That also ties into not exactly a cliché but something I've seen nonetheless where Rome is treated as this sort of unbreakable, unchanging stagnant pyramid where the typical issues you find in any empire or the usual roman ones are either glossed over or become trapped in this time loop of going on forever without being changed or challenged in any meaningful way, like the use of slaves in Roman society, in a TL where the Roman Empire keeps on trucking without major issues is absolutely something that would need to be addressed by emperors with the consequences of having something like a economic crisis or severe opposition from people who profit from slavery or something like the major religious disputes that even before Christianity were pretty important but became even more when the majority of the empire became Christian, iirc something like the failure of emperors to deal with stuff like the Cacheldonian disputes or heresies like Arianism or Donatism were key causes in making territories break away more easily ie: Arab invasions as the mostly Orthodox Cacheldonian Greek people of Anatolia, Balkans and Antioch were in dispute with the Syriac church, Coptic church and Nestorian church over the nature of Christ meaning that these people actually preferred the rule of Arabs instead of "living under heretic Emperors".
