You can get pretty creative with a POD in the early stages of the Migration Period.
You can keep some Germanic kingdoms that faded away in OTL afloat (Suebian Galicia, Ostrogoth Italy, Visigothic Spain, Avar Pannonia), have tribes/people settle and integrate elsewhere, (have the Vulgars go north, or the Magyars go further south).
If the POD is pre-Islam as well you can also go crazy with the Arab conquests/expansion, make their push through the Caucasus successful, and push the Khazars into Europe, while halting their advance into Iberia in the West. You could go cray on the religious end with surviving heresies movements that faded in OTL, or new splinter movements within Christianity and Islam (extra points if Constantinople falls early). And you can create whole new religious movements; it is not too late to keep some paganism alive in parts of Northern and Eastern Europe, which can have their own ethnoreligious implications.
There are some places, however, which will be very hard to change into completely different “nation-states”. Ireland is already “Ireland” by this point, even if they get their own national Church/Faith and evolve differently it is likely seen as a version of "Ireland" unless it's part of a larger nation and wholly assimilated. Similarly, Scandinavia, in general, would still be considered “Northmen/Danes” and no matter the level of migration that you have, it will likely remain Germanic-speaking. However you can keep Scandinavia pagan or split into different states than OTL.
Any early enough POD can give some interesting results.