... Only without the Rhineland, being in Prussian hands and all.
That's the biggest obstacle to any call for a tripartite division of The Germanies, so far as I can see: Prussia controls two large regions that would play merry hell on the third part's security and usefulness/border to an exterior patron. Perhaps you could get a revolution in the Prussian Rhineland leading to them breaking off from Brandenburg proper? The question is how to bring that about
Weren't there '48 Revolts in the Rhineland? Could've sworn I read that there were. The question of course whether it was on this forum or in an actual history book. But if there were, could they not simply be successful enough that it takes the Prussian govt a bit to put down or that it seems like the exclave is in danger of being lost.