I guess it ends up another part of Poland like Pomerania is. It probably speaks a unique dialect of Polish and has a very distinct regional identity, but otherwise is an integral part of the Polish state. It might lead to Poland developing more of a naval tradition than OTL.
I just don't see it being Slavicised without the Poles being able to conquer the place, which if I recall, medieval Poland was never able to and often were the ones having to defend against Prussian raids. And if the early medieval Polish state couldn't do it, I don't see any of the pre-Christian Slavic tribes being able to either. Maybe get the Mongols to utterly trash the place and/or have a worse hit of the Black Death (while Poland still has comparatively low death rates from it), and events so happen in a few waves of settlement after these disasters the population is mainly a mix of Poles who assimilate the Prussians over the years.