How would the Prussian State develop with a large polish population?
A few decades later, Prussia wouldn't be (as) capable of appealing to the kleindeutschland faction (if it's around, of course) among those favoring German unification. Saddled with a large Polish population, one that would demand rights and privileges commensurate with their share of the nation, Prussia wouldn't be able to simply displace Austria in terms of German leadership.
Further, the Poles were able to demand (and for a while maintain) a "Congress Kingdom" autonomous within Russia OTL. Prussia is much less capable ITTL of resisting Polish demands, and in any case would be fearful of Russian (or perhaps Austrian) "intervention" in Prussian Poland ostiensibly to prevent disorder from spilling over the borders.
It's an interesting question of what this shiny new Polished Prussia* would demand from Austria were war to break out between them. Would Galicia or parts thereof be more attractive ITTL?
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*I know, I know. I deserve to be pun-ished.