What if Licinius beats Constantine and makes Christianity illegal again? Wouldn't it be cool if pagan France and pagan England were fighting the 100 Years War?
Well if the Roman Empire dissolves in that TL, perhaps there would some nationalistic differences. Maybe "pagan" France would be practicing Roman Polytheism, while pagan England, if indeed it does get colonized in the past by waves of Angles, Saxons, and Danes, would probably be espousing a codified form of Norse Polytheism.
What would it be like when the pagan crusaders attack the middle east 800 years later?
We would not have the same motivation for the crusades in this TL. Also, without the earlier Christianization of the Roman Empire, and the destruction and suppressment of both polytheist religious and scientific knowledge as a result, maybe Europe wouldn't be in quite the same state as it was at that time.
How would China respond to pagan Marco Polo?
Marco Polo might not be as famous for the same reasons as he was in OTL.
Would the Spanish being pagan make first contact with the Aztecs go differently?
Other than thinking that human sacrifice is primitive and barbaric, the mainly Roman God worshipping Spaniards wouldn't have a problem with incorporating the Native American pantheons. Other than that, there is no guarantee that the Native American cultures wouldn't suffer genocide and enslavement.
Finally, when the US rebels against Britain, would we still have freedom of religion, or would paganism be the state religion?
Past pagan cultures generally allowed freedom of religion. America wouldn't be any different.
Aside from these, I think there would be far more changes to the timeline than just national religions. Far more.