The Sami gradually expand southwards into Sweden and Norway, pushing the Norse down into Denmark, Germania, and Poland, and increasing conflicts that the Norse have with the French and English. They also expand eastwards into Finland, assimilating the closely-related (culturally) Finns and Karelians. For fun, let's also say that this results in the gene that gives the Sami people their almond-shaped eyes (which is actually different from the one that results in similar eye shapes in East and Central Asians) despite otherwise looking like pretty standard North Europeans is also propagated extensively (I know this sounds like some weird white nationalist shit, but I just find it very interesting I guess and don't intend to imply any inferiority, superiority, or even preference with it).
Meanwhile, in England, the Anglo-Saxon kindoms Remain separated, and actually splinter even further. Despite this, Wales is also conquered and assimilated into just another Anglo-Saxon kindom, maybe with slightly more Brittonic linguistic influences. Cornwall, however, still stands against the tide of Germanic invaders (and against the second tide in the form of Norse Raiders when they come). Eventually, they even turn the tide and begin slowly creeping over southern England, gradually conquering more and more land. The people of these lands are slowly assimilated into Cornish culture and learn the Cornish language, which, though both are similar to the Welsh culture and language, are still distinct and have some very prominent differences.
Then, the Black Death hits. Most of mainland Europe is effected, but Scandinavia really... isn't, and the British Isles are relatively sheltered. With a large portion of mainland Europe gone, even more North Germanic and some Anglo-Saxon people migrate back to mainland Europe after the plague passes. With this, the Sami and Cornish are free to expand even more. The Sami end up with Karelia, Murmansk, Finland, and all of Scandinavia except for Denmark. The Cornish end up with pretty much all of OTL England, with a sphere of influence stretching out into Denmark, France, Brittany, and the entirety of the British Isles.
The push of the wanked Sami and Cornish result in a lot of displaced Norsemen in Northern Germany, Poland, the Baltics, and OTL Scotland, as well as a lot of displaced Anglo-Saxons in France, Denmark, and OTL Scotland.
The result is that, in TTL, Germany and Poland have a lot more Norse/North Germanic cultural influences, especially linguistic influences in the case of Northern Germany.
Brittany in TTL is semi-Autonomous, though France and Cornwall jockey for influence in the region and occasionally outright conquer it.
In France, a weird bizzarro version of what happened to the English culture and language in OTL happens to the French culture and language in TTL (only to a lesser extent). Instead of English language and culture receiving a lot of French influence from the conquering Normans, French language and culture receives slightly less Old English influence from the migrating Anglo-Saxons.
What in OTL is Scotland is the weirdest and most transformed of them all. Having received a lot of both Anglo-Saxon and Norse migration to a land with a lot of Gaelic Celtic cultural influences, it develops a weird sort of twisted version of Scottish Culture only vaguely resembling that of OTL, and the language of the region is even more convoluted, especially when one takes into account the massive Brittonic Celtic cultural influences from being part of the Cornish sphere of influence. In TTL, it eventually comes to be known as Nornland, and its language is one of the most difficult and confusing languages to learn, though French is pretty close behind it.
The Sami realms (OTL Norway, Sweden, Finland, and Karelia/Murmansk) eventually break up into a bunch of smaller, culturally distinct nations, though they still share quite a few similarities.
Meanwhile, Cornwall eventually unites with Nornland into a weird, sort of inverted but also sort of completely fucking insane version of the UK, complete with a period of dual monarchy before an Act of Union obviously imbalanced in the favor of the Cornish, and with both countries picking on Ireland before eventually just Conquering it.
I was going to do a whole segment on religion (lots of religious uphevals and shit), but this was already getting too long.
Needless to say, this was supposed to be technically somewhat possible, but is not in any way realistic. I hope you enjoyed me doubling down and wanking not one but two obscure ethnic groups.