An earlier formation of powerful Slavic states concentrated around OTL Germany may also allow this Slavic Millieu to absorb/repel the Magyars, leaving the Pannonian basin to be Slavic as well (Hungarian Slavic hybrid language, maybe?).
Arguably, OTL Hungarian before reforms was packed with Slavic influences as English was with Romance, so any strenghtening of Slavic presence in Pannonia would leave this region to total slavicization, like OTL proto Bulgars got slavicized.
So you'd get something like Slovak with lots of Ugro-Finnish loanwords.
Also powerful Slavic state concentrated around OTL Germany could repel and destroy the Avars (who were there before Hungarians) like Charlemagne did IOTL all by itself.
However, the POD you gave isn't likely for Slavic Germany, IMHO Franks falling to muslim invasions and Sorbs, Bohemians and Obodrites pushing into Germany (the influx of Frankish exiles might also help them create stable states) is better POD for that effect.