POD: Slavic migrations settle a bit further into northern Germany, displacing the Saxons in particular, and establishing themselves upon the coast of the North Sea. Raiding and invading brings them westward along the Frisian coast to the lower Rhine.
Can we get a similar viking dynamic from those slavic peoples? IIRC, there was cross-Baltic raiding going on, so extending that to the North Sea and beyond doesn't seem too far fetched.
The challenge would seem to be that the Franks might be much more capable of responding to the challenge, and of taking the fight to the raiders. Then again, it's not too hard to imagine the Frankish empire getting hobbled a bit.
So, is it possible to give the Norse some competition this way? And just how unlucky would the British isles be ITTL?