I'm working on something similar, albeit coming from a very different direction with a much earlier POD. I'd be interested to hear your thoughts on the extent your (presumably Saxon-centered?) empire would be able slash willing to syncretize with neighboring religions. Not to mention wanting to get at your sources! &)
A lot of the initial pressure for syncretism is going to be due to the close proximity of the Saxons and the various Wendish/Polabian Slavic groups, such as the
Obotrites,
Lutici/
Veleti,
Sorbs and
Rani, who are going to be early additions to Saxony's expanding hegemony, as well a the Bohemians/Moravians who are going to be brought into Saxony's sphere of influence initially thorough a marriage alliance against the Bavarians and Poles but become increasingly subordinated in the aftermath of the disruption caused by the Magyar invasions.
In addition, as I am working on the premise that the Saxon Kings are going to increase their power and influence within the Saxon polity over time, the Saxons are going to embrace some of the institutional practices of Slavic Paganism and blend it with their own traditions. One of the main differences between Germanic and Slavic paganisms is in terms of priest structures. Germanic pagans had looser and less defined priesthoods with Kings and local rulers presiding over many religious ceremonies. In contrast the Slavic pagans had a more defined and separate priestly class, often centred around temple complexes and holy sites such as
Radgosc and
Arkona. Over time the Saxons co-opt this priestly structure to better help rule their Slavic subjects, in sharp contrast to the Christians IOTL who violently suppressed them and their culture, and eventually expand it within their German territories. Over time the Saxon Imperial Cult, at least in part, emerges as a result of the blending of these two traditions with the King serving as High Priest and the clergy directly subordinate to him.
Finally there are also demographic factors that will lead to a blending of different traditions across the Baltic Sea. The Saxons briefly have a period of supremacy where they end up going Alexander the Great on a Europe reeling from the Magyar raids, Viking Age and Muslim conquests and are able to conquer everything between Northern France and Lithuania, albeit very briefly before the whole thing comes crashing down the moment the King dies. The result of this is that large numbers of people from the various cultures are moved around, either as a result of being taken as slaves or soldiers and allies being rewarded with land grants in conquered territories. In addition there are also the various Scandinavian people who traded, raided and established settlements throughout the Finnish, Baltic and Pommeranian coasts. When Saxony and the various successor states that arise in the aftermath of the post-Saxon chaos begin to centralise and consolidate their position the state religion is going to blend elements of the different pagan traditions, as well as elements of Christianity and Judaism (I have some interesting ideas for how Judaism is going to develop over time), together.
I'm also thinking of having the
Livonians become an established regional power incorporating territory inhabited by Finnish, Baltic and Slavic people as well as maintaining contact with Scandinavia and Saxony, which would provide ample ground for syncretisation to occur.
In terms of sources I'm currently in the browsing Wikipedia articles stage although I have also brought a few books on the Christianisation Europe, though
the one I am currently reading seems to be focused more on Hagiographies than sociological and anthropological information of the pre-Christian societies in question. I am trying to find more stuff on Slavic, Baltic and Finnish religion and culture but unfortunately a lot of the stuff I'm looking for is behind paywalls or are out of print academic books that cost far more than I am willing to pay for.