The Holy Empire of the Labians or the Elbe Slavs conquer the Germans and Slavicization them.
Does anyone have an idea how to achieve this?
Historically, the Elbe Slavs, placed between Poland and Germany, were the arena of clashes between their neighbors from which the Germans ultimately emerged victorious, conquering and Germanizing the lands up to the Oder River.
How to make the "Labians" (literally Elbians) kept their independence as well as conquered the Germans?
Tough call
It is indeed a tough call. I think
@Buba's POD and general scenario would suffice to cripple the Germans and thus allow for the consolidation of a Polabian state covering, essentially, OTL's (modern-day) "East Germany".
I don't think that such a POD would suffice to bring about the annihilation or displacement of the Germans, however.
I'd start with the Franks failing to conquer Saxony. Followed by the Franks dividing their empire and/or into smaller bits. Hence no Frankish/Saxon/German steamroller, but weaker, divided neighbours to the west.
Then the Hungarians - can be Bulgars or Pechengs, any sort of steppe nation will do - set up shop not in Pannonia, but along the upper Danube in what today is Bavaria. They wreck the entire neighbourhood weakening everybody, causing westward flight of people and institutions, for two or three generations, before being wiped out or assimilated.
To this add Viking raids from the north and the Polabian encroachment.
This sets the stage for westward expansion of the united Polabians in the early Xth century.
The major issue is that this is too late. Basic truth that has to be recognised: the Germans were in closer contact with Rome, and in a better position to (re-)build stuff on the ruins of Rome's power structures. Thus, you get Romanised Germanics founding post-Roman successor kingdoms, and less-Romanised Germanic states further North. But the latter could and did still use Rome's legacy very effectively. The "Christianity bonus" was theirs, too, since early conversion really made a difference.
We should also consider that any Hungarians or Bulgars or whatever are unlikely to just cross the Alps en masse. So they either stay in Pannonia, or they screw over *Czechia, or... they move into *Bavaria....
through *Poland and Polabia. In the process screwing those regions over in a major way, and negating any advantage it might provide for the desired scenario.
So what to do? I think the required POD, truly at the very
latest, would be "no Clovis". Or rather: instead of Clovis, the Franks get some guy like Redbad, who decides "Screw all Christians, I hate them!" and creates a conflict with the states to his South. That should create a nice mess.
The end result is that Southern *France (extending further North than in OTL; I'd say to the Loire) consolidates as a Post-Roman, Christian region, whereas the Franks remain pagan (and more decidely Germanic, too). They get crushed both by the Christians and by other pagan peoples (e.g. Frisians, Saxons, Thuringians), which in turn causes those people to focus their attentions South(-West), where they can hit and loot the Frankish domains.
This general Germanic warlord area (between the Loire and the Elbe) stays a big, violent mess for a while. This allows the Polabians to consolidate, since they don't face any pressure from those aggressive Germanic raiders who were so pushy and bothersome in OTL.
The most plausible scenario is that the Polabians end up doing what Charlemagne did. They conquer various neighbouring Slavic peoples
first. Thus, they establish a "Polabian Empire" that includes the proto-Poles and proto-Czechs and whatnot-- all the way up to the Vistula in the East, and extending so far South as to include the early Croats, too. So you get the basis for a "Slavia" instead of a "Germania". And then, like Charlemagne did to the Polabians, in OTL, they push West and subdue the Germans living between the Elbe and the Rhine, and they make that region thoroughly Slavic over the span of a few centuries.
Later on, this Slavic empire may well fall apart into multiple states, but there is the basis for a strong cultural entity here. Either this empire or its Westernmost successor state can push yet
further West (just as the OTL Germans pushed further East), conquering the Franks. With the effect that this Slavic cultural sphere pushes its South-Western border to the Loire.
