I just want to say I'm working on some features related to western Slavs. Of course there are dynasty placeholders, but that's not only I work on...
New religion
When we have "blank pages" in this timeline, I'd like to introduce history of
Slavic Christianity. I've stated it once, but I'll do it again:
* In about 4th century Christians start to explore the most western Slavic states (White Croats, etc), but yet not many conversions. While of course, religion will be known and will exist in there.
* After great migration of tribes, especially Slavic ones, they settle borders of the Roman Empire. Shouldn't be strange to have Christian missions to them, and some bishopric for the purpose. Of course it would be before settling Serbs and Croats in Balkan region, so as we have strong position of Dacian and Sarmatian. That's why it needs to be some place near the border, but on the Slavic territory. So I'd like to see first Slavic bishopric in Ostrihom, and have it as their center of local Christian society.
* Because no schism yet, missions come from both western and eastern part of the empire. Some monk named Kyrillos (Cyril) created special alphabet for the liturgy, based somewhat on original Slavic letters and Greek (Glagolitza). After Serbian will move there will be prepared the new alphabet for them, more based on Greek (Cyrillic).
* When Samo tooks his reign, he takes persecutions on Christianity, as possible danger for him.
* Croats and Serbs are moving out to their new places, taking the rite with them, of course. Ostrihom lose it's importance because of Samonids, whilst the liturgy (and it's alphabet) give Slavs some culture boost (in western standards). Slavic Church still functioning, though.
* Mojmirids would be restorers of Great Moravia - and Slavic Christianity. That's how I'd like to see them. Ostrihom rises again. But of course we have Avar state in Pannonia, and incoming Magyars... Moravian state collapse (again), Ostrihom taken by Avars.
* Now, the situation in Christian group. Schism is close, Pope don't like the new liturgy, it's getting abandoned... at least for him. Slavic Christianity becomes a heresy, still highly influenced by Orthodox rite.
* Cyrillic spreading far than Serbia, changes in culture, etc.
The new religion would be:
Christian / local variation / (...) / (may be Barbarian ascendant)
About soul, it seems hard for me to decide, which one would be better for it. "State" I was looking at, but it doesn't fit well with proposed scenario (not much of rulers' support). But also "traditional" I thought, yet doesn't feel good enough. The last one would be "scholarly", which might be suitable. But if you want to say about it, I'll listen. But don't expect it to be "populist", "matrial" or "messianic" (unless it would be followed by Samonids... what I doubt because of Shaytana's vision prepared for them).
Who? Serbian for sure. Actually, all Slavic Orthodox should be. Croatian would be nice instead of Catholicism, but some political reasons stay on the way. And another state would be... Check below, or look for the hint from the history above
God's Shift
Situation doesn't feel good with OTL beliefs in specific regions. That's why I'd like to "swing"

-
Rod - this god wasn't supported in Poland, it's more eastern god, actually. Might yet work with southern Slavs.
-
Svarog - oh, yes, the Solar one. It's rather not only Solar deity we would speak about (Dazhbog would be either), but yet Numahr decided he should be the one, so... Tracking him we can found ourselves in Greater Poland (where he might stay), Pomerania (forget about Rodnovery), and Sorbs or Polabians (Schwerin - probably slavic Swarzyn, from Svarog of course).
-
Dazhbog - the second Solar god. I might see him as another emanation of Sol Invictus influence in region, even better than Svarog, though. But to the point - doesn't feel the best choice for Moravia. Rather for Sorbs/Serbian and Croatian (and similar, like Bosnian or Slovene). And for Eastern Slavs (Mazovia or Vistula considered).
-
Perun - one of the most popular gods. His cult was between all Slavic states. But as his supporters we could consider Polish tribes (Silesia, for example, after we take Rod from there, or Vistula might stay), and Kiev.
-
Svetovid - the hardest one. When checking his seen cult, we should stay only in Polabian/Pomeranian region (Rugia was center of his believers). Hardly seen in Mazovia, methinks.
There are also other gods, like Radegast/Radogost and Svarozhic (Svarog's son). But we find them both too similar, also with Dazhbog. That's why they're discussed to be different names of the same deity. Of course it's not sure at all. Anyway, Svarozhic faith was centred in... Radogost village (sic!

), so seems suited for Polabians.
So let's go straight to the point, what I see is (my
sure choices underlined):
*
Polabian -> because Dazhbog should stay for Sorbs, then
Svarog stay
*
Sorbs ->
Dazhbog, like stated above
*
Pomerania ->
Svetovid; Stettin should be given to Svetovid or Dazhbog I think, rather than Svarog
*
Polans ->
Svarog
*
Silesia -> instead of Rod, might be
Rodnovery or Perun (if so, then not in Vistula)
*
Kuyavia -> hard to decide, Rod or Dazhbog maybe - for your consideration
*
Vistula -> Perun as is, or can be Dazhbog (Solar rivalry with Polans? only if not in Mazovia)
*
Bosnia ->
Perun
*
Slavonia -> maybe Svarog instead of Rodnovery? Not sure (Tvarog in Steiermark seems to come from his name, so it's close there)
Now, of course Nitra stay as is, I don't touch Veles cult.
Still
Moravia left. As the matter of fact, I'd like to see it
Slavic Christian. But if not, then for historical reason I'd rather take Perun or just
Rodnovery instead.
EDIT: sorry, it's Kuyavia, not Mazovia... while I find it more proper.