Well, the "Romans" part of
this post gave me an idea
What if, during the Crisis of the Third Century, the Gallic and Palmyrene Empires were more successful in expanding against the central Roman government, but after war weariness and seeing the difficulties of the unified Empire in the preceding decades, established an accord with each-other and essentially just established the Eastern and Western Roman Empires via a different path than the Diocletian path?
Originally I was just going to make minor edits, because frankly it just amuses me to imagine the split being established differently like that but then going on to otherwise go the same. However that was kinda boring so I changed things up a bit
Might do some more with this scenario, its a bit of a change from the more modern/ideologically oriented stuff I tend to do
The Roman Empire emerged from the West, but by the Third Century, the Western half of the Empire was decidedly the weaker of the two, on paper at least. It had a smaller population than the East, a less dense and more spread-out population, a less developed economy (as compared to the East, with older and more urban population as well as more established trade routes), and a longer frontier. Indeed Postumus' revolt, which established the Western Empire, was in a large part done simply due to the difficulty of the central government to protect the frontier in Gaul during the third century crisis
Given the weaker position, and the strains that the entire empire was facing, it could be seen as something of a surprise that the west survived over a thousand years longer than the east. A major reason why the west was able to survive, despite its weaknesses, was a tolerance and pragmatic openness to working with foreigners. In the latter parts of the fourth century, the entire frontier saw pressure from largescale migrations of Germanic peoples, themselves being pressured by the Hunnic invasions.
The Gothic Affair is exemplary of the contrast between the east and west on that matter. The Goths were one of the Germanic tribes pushed by the Huns to the borders of the empire, to the Balkan territory of the eastern empire in particular. The Goths didn't come with intent of war, and merely sought refuge and the ability to settle and serve the empire. The Emperor in the east did reluctantly grant the request, being busy campaigning against the Sassanids, but at the Emperor's order, the officials tasked with managing the settlement of the Goths had subjected the Goths to many abuses and exploitations, with the goal of spending as few resources as possible on the Goths while extracting as much profit as possible from them. Things eventually came to a head, and the Goths rose up in open revolt, pillaging across the northern Balkan territories. The Emperor in the east, in a fit of rage, ordered the execution of the entirety of the Goths, and began to mobilize an army for that goal. But by the time the army had arrived the Goths were gone from the east
The Emperor in the west sensed opportunity. He was facing a war on the frontier himself, as well as famine and plague in parts of the empire, and a rather irate and dissatisfied aristocracy, putting him in a rather insecure position. The Emperor watched with interest as the Gothic Affair erupted in the East, and reached out to the leaders of the Gothic rebellion, with an offer - just leave the east, and fight for the west instead. The west was near-bankrupt, but could promise fair treatment and land on the frontier, as well as some portion of the spoils of war there. The Goths wearily agreed, their army played a key role in turning the tide on the frontier, the Emperor stayed true to his word and treated the Goths as countrymen no less Roman than Italians, and the Goths became stalwart western loyalists and citizens. The move was controversial among the western aristocracy, with negative sentiment towards the newcomer migrants and the tolerance shown towards them, but the Emperor argued against the backlash, citing history of the Social War as an example of how exclusivity in regards to citizenship could create major issues for Rome, and aristocratic dissent was, if not ended, at least made less of a threat, with the Emperor's position strengthening due to acclaim of the victory on the frontier
The western empire continued to experience strife within and without, but the leadership came to take a stance of acceptance, accommodation, fair treatment, and integration of outsiders - provided they were willing to be brought into the system - that allowed for a flexibility and vitality that allowed the west to punch above its apparent weight. Meanwhile, in the east, a short diplomatic crisis emerged, with the eastern Emperor demanding the heads of the leaders of the Goths to pay for their rebellion, and threatening to go to war over the dispute. But the Emperor in the east died before any such war could emerge. A new border crisis soon occurred, with the Vandals also fleeing the Huns, crossing the border and begging the eastern Emperor for refuge - he instead declared the Vandals invaders, and led an army against them, dying on the field of battle in defeat, while the Vandals in righteous rage poured into the Balkans, sacking many cities as they went. Following eastern Emperors restored friendly relations with the west, but further hardened their stance against the tribes on the border, and even when the east eventually relented and reluctantly tried to adapt by giving tribes foederati status, relations between the eastern leadership and the newcomers were very prone to conflict. Indeed, the empire in the east was only ended during the reign of Julianus due to the Roman aristocracy refusing to pay their Germanic troops and generals, leading to their revolt
The eastern empire had also faced conflict over religion that helped lead to its fall. It was the more Christian half of the empire, and had eventually ran into issues with emperors converting, attempting to convert the entire empire, and then facing backlash over this from the Pagans and from various competing sects of Christianity as well. By the late era of the eastern empire, conflict had caused several outright rebellions as well as see-sawing back and forth between Christian and Pagan emperors. The west, meanwhile, also had sizable amounts of religious diversity (with Christians, Jews, and 'foreign' Pagans), but the traditional Roman civic religion was still a solid majority, and the emperors felt less threatened (or bothered at all) by the existence of religious minorities, largely feeling content with integrating them into the state religion or just leaving them alone. The lack of religious wars was thus one more advantage for the west, one more headache avoided that the east had faced
The empire in the west proved more durable than the east, but nothing lasts forever. The west, too, would decline over time, shrinking and facing more and more external competition. In 1561, the west, too, would finally fall for good. But the empire had an expansive legacy in many ways that survived the fall of the empire (indeed, the Turkic peoples who conquered the empire styled themselves as "Rumans"), and that legacy is still seen even today