It's hard to imagine how you could make a religion adapted to the need of a city-state culture a world religion. They are not so well-suited for larger empires which need legitimization for their state and emperor, either in an emperor cult or in more universal religions. For your scenario to work you'd have to make a world composed primarily of city-states. I'm not sure if that could work in the long run.
Rome's original religion was inherited from the Etruscans, so it was practiced amongst other city-states as well. Also, it was heavily observed by the rural populace which indicates it was more universal than its given credit for. Religions are only as complex and advanced as the culture that espouses them, so if some obscure Judean revelatory cult can be adapted as the state religion of an empire, then so can a former rural animistic tradition. Christianity didn't spread as far as it did because everyone thought it made sense or found it rewarding in some way, although a few probably did, but because in the years before it rose to power, they spread themselves across the Empire, maintained contact with one another, and developed their own internal organisation and hierarchy, etc. Even by Constantine's time, their were still far more followers of the polytheist traditions then the new mystery cults. And even after Theodosius' time, there were still lots of secret polytheists or double-faith types that craved for the old days. Granted, the Imperial Cult could have been expanded on.
Even in Hellenistic times new leadership cults were institutionalized centred on the monarchies of the Hellenistic empires. But such leadership cults are limited, something more universal was called for, I think, though hardly inevitable. It might not have to be Christianity. Given the contacts with India in the Hellenistic periods and the existence of Hellenistic states in Pakistan and Afghanistan you could argue for a POD with one of the Hellenistic monarchies choosing Buddhism as a state religion. If that would hold you'd have a pretty radical break with OTL. And my gut feeling says that Buddhism could co-exist better with both traditional beliefs and the intellectual developments in philosophy (which where also present in India).
It's hard to imagine how you could make a religion adapted to the need of a city-state culture a world religion. They are not so well-suited for larger empires which need legitimization for their state and emperor, either in an emperor cult or in more universal religions. For your scenario to work you'd have to make a world composed primarily of city-states. I'm not sure if that could work in the long run.
Even in Hellenistic times new leadership cults were institutionalized centred on the monarchies of the Hellenistic empires. But such leadership cults are limited, something more universal was called for, I think, though hardly inevitable. It might not have to be Christianity. Given the contacts with India in the Hellenistic periods and the existence of Hellenistic states in Pakistan and Afghanistan you could argue for a POD with one of the Hellenistic monarchies choosing Buddhism as a state religion. If that would hold you'd have a pretty radical break with OTL. And my gut feeling says that Buddhism could co-exist better with both traditional beliefs and the intellectual developments in philosophy (which where also present in India).
Indeed, why should a religion that splintered from a tradition that was developed by desert nomads be more suitable as a universal religion than a long established and complex tradition espoused by those that conquered numerous countries? I would think the spiritual beliefs of an isolated rainforest tribe could potentially become a world religion, given time and development and with more equal competition.
Also it would be naive to think that Christianity provided any sort of unity to the Empire. What about the dissent between the Trinitarians and the Arians? If Christianity united people all over the Roman Empire, then why were there citizens whom collaberated with Vandals in Africa, and with the Visigoths in Aquitaine and Spain, mainly because they shared the same brand of Christianity with the barbarian invader? When the remaining traditional polytheists bemoaned that the sacking of Rome and the constant operation of barbarian armies within Roman lands was the consequence of abandoning the Old Gods of Rome for a provincial cult, Saint Augustine of Hippo wrote in his work, "City of God", that the attack on the city didn't matter in the grand scheme of things, since the "Kingdom of God" would always prevail. Comforting to a devout christian maybe. But totally irrelevent in the mind of a traditional who cared about the here and now, rather than someone elses vision of the afterlife. There was more to the causes of Rome's downfall than establishment of Christianity, but it still contributed its fair share of trouble, given its fanatical "One True Faith" philosophy.
I think you are chalking too much of the fall of Rome up to Christianity. By the time Constantine made it the state religion, Rome was already very weakened. The old religion didn't provide stability any more than the Christianity did (Crisis of the Third Century springs to mind)... so not adopting Christianity would not have meant a great deal in the long run (in some respects). Rome still would have fallen eventually, and the "here and now" old religion followers could not have stopped it.
Christianity managed to create an institution that outlasted the Roman Empire.
I think the key to creating some kind of lasting paganism is to beef up the cult around the Emperor. Preferably to something that encourages an orderly line of succession.
But a religion designed for nomadic goat herders is better suited to be a world religion?
Bactria was ruled by a Hellenistic monarch and the religion was Buddhist.
I think you are chalking too much of the fall of Rome up to Christianity. By the time Constantine made it the state religion, Rome was already very weakened. The old religion didn't provide stability any more than the Christianity did (Crisis of the Third Century springs to mind)... so not adopting Christianity would not have meant a great deal in the long run (in some respects). Rome still would have fallen eventually, and the "here and now" old religion followers could not have stopped it.
I dunno. Hinduism and Buddhism seem to be managing just fine without an imperial cult.
I really don't see how Monotheism is an advanced type of religion. Its no more mature nor ground-breaking as the idea of highly evolved spirits that influence the course of the universe. Religio Romana, Hellenistic, Celtic, Germannic, Slavic, Egyptian and other spiritual traditions anywhere else weren't going out of fashion. They were purposefully repressed. Any of those could have survived with an evolving culture.
As for Neoplatonism, they weren't monotheists per se, thats just wishful thinking by christians. Neoplatonists were actually "Pantheists". Thats to say that while they believed that there was a single supreme being or entity, they believed that this creature would present itself in various forms and they felt it was only right and proper to pray to it accordingly.