It's not even very clear what a Reformed Religion would look like. In context, it seems to need to be a religion that can hold its own, and command the loyalty of the populace, even in the face of missionaries from other faiths. It needs to have some sort of way to lend legitimacy to its rulers (which was a big reason for the success of e.g. Christianity in making in-roads; get some water sprinkled on your head and you become an internationally recognized big shot). And it needs to offer access to the broader trading zone (this was another advantage; Christian merchants and rulers might do business with a pagan, but they were much more comfortable doing business with a fellow Christian).
So you need popular support, and a sort of missionary culture. Pagan faiths generally didn't have a universalist missionary ethos; their religions tended to be tied up with their tribes. They also generally didn't have much in the way of standardized mythology; don't be fooled by the ready availability of modern books on e.g. "Norse Mythology," there were wide variations in the various myths involved, and general lack of consensus on a lot of things.
Critically, I think a lot of the "Great Ruler decides to institute reform and revitalize paganism" ideas, such as the ones modeled on Julian the Apostate, are fatally flawed. A religion needs to have popular legitimacy if it is going to provide political legitimacy. Random ruler saying "we worship this way now, and here are how our gods want you to do things" is just going to be ignored by the majority of the peasantry. Christianity took centuries building up that kind of legitimacy before Constantine, and converts could take advantage of that preexisting cultural, theological and physical infrastructure. Islam was a bit more sudden, but even there, Muhammad started as effectively a street preacher before he moved to Medina, and was able to build on preexisting exposure to monotheistic religions.
So you need popular support, and a sort of missionary culture. Pagan faiths generally didn't have a universalist missionary ethos; their religions tended to be tied up with their tribes. They also generally didn't have much in the way of standardized mythology; don't be fooled by the ready availability of modern books on e.g. "Norse Mythology," there were wide variations in the various myths involved, and general lack of consensus on a lot of things.
Critically, I think a lot of the "Great Ruler decides to institute reform and revitalize paganism" ideas, such as the ones modeled on Julian the Apostate, are fatally flawed. A religion needs to have popular legitimacy if it is going to provide political legitimacy. Random ruler saying "we worship this way now, and here are how our gods want you to do things" is just going to be ignored by the majority of the peasantry. Christianity took centuries building up that kind of legitimacy before Constantine, and converts could take advantage of that preexisting cultural, theological and physical infrastructure. Islam was a bit more sudden, but even there, Muhammad started as effectively a street preacher before he moved to Medina, and was able to build on preexisting exposure to monotheistic religions.