Now I know that Christianity had in fact absorbed certain pagan elements into itself, but what would a religion that more fully blended Christianity with the various types of Paganism across Europe look like? would it be something similar to say Sikhism were it starts off as a sort of blend of two faiths that gains its own identity over time, or would it be something different?
So, back to the OP's original request, I had this in mind for a while. I was rereading some excerpts from Julian's "Hymn to King Helios" and it is striking how some theological ideas he put inside it were outright hijacked from the Christian Theology. For instance, Helios is the supreme mediator between God and mankind, in a fashion that is strikingly similar to Christ as "Logos". Also, Asclepius, his son, is labeled as "Universal Savior". In a book about Julian that I read recently, the author basically blames Julian's refusal of the Christian religion on the "bad teachers" he had in his youth, all of Arian faith and thus uninterested in the finest theological debates (disclaimer: I am reporting the authors ideas). So, assume that this is (ate least to some degree) correct. Let Julian have "better" teachers in his childhood exile, so he becomes a more convinced Christian young man, but leaving his love for the classical greco-roman culture intact. Assuming that he becomes Emperor on schedule and he still goes on with his reform, could this alternate reform be a real syncretistic attempt? I know it might sound crazy, but let's try. There is One God, who decided to reveal Himself to the Jews, but since He loves us all, he sent some of His Angels, disguised as Gods, to take care of the rest of mankind, the most powerful being Helios/Sol Invictus/Apollo. Then he sent His Son to live and die to redeem our sins. It happened in Roman Empire at the time of the first Caesars, for a reason: with only its own pietas and wits, the greco-roman world had achieved the highest peak mankind alone could achieve, and it was now ready for the final "salvation" step... a lot more than the original Chosen People. We may also see Christ taking its rightful place in Heaven... where always it had belonged: as Helios/Sol Invictus/Apollo/Christ. After all, the solar aspects of Christ are some of the reasons (or better said, justifications) why Christmas is on December the 25th. The sacrifices to the Pagan Gods could be kept with the same "escamotage" Catholics pray/give offers to the Saints: to get their intercession with God on a specific matter, not because they have power per se. In order to work it would require a longer reign of Julian and a decisive victory against the Sassanians, and the endorsement of some very influential bishop (I am thinking of Gregory of Nazianzus, who was schoolmate of Julian in Athens). Would not work, probably, but still...