My proposition of POD:
Elisabeth of Russia lived much longer than in OTL so Russia didn't switch sides in Seven Years War, Prussia collapsed. Ducal Prussia was taken by Russians who exchanged it with Poland-Lithuania for Courland and eastern Belarus, Silesia went to Austria with exception of strip in northern part given to Saxons to give Saxony common border with PLC. Sweden regained Pomerania. Hohenzollerns lost royal title and everything they saved was Brandenburg. Now PLC has much higher chance to survive-without double-dealing prussian promise of alliance and with Wettin king residing in Dresden there are low chances for something like Third May Constitution which irritated Russia so much in OTL, so PLC survived until revolutionary wars as russian vassal and instead radical constitution slow reforms tolerable for Petersburg continued. Let's say some Napoleonesque figure emerged (POD is before Nappy's birth so he may be butterflyed)-now Russians are forced to allow rearmament of PLC if they don't want French revolutionaries to conquer Poland. At the same time alter-Napoleon created new kingdoms and duchies in Germany, so number of German states decreased. After alternate Congress of Vienna new order in Germany is estabilished:
-large or medium sized kingdoms: Hanover (which is enlarged-Rhineland is added to it so there are no more mess of microstates in Western Germany)-in personal union with Great Britain, Saxony (containing Thuringia also) in personal union with PLC, Bavaria, Baden, Wurtemberg, Mecklemburg and Brandenburg.
-Pomerania stays in Swedish hands, Schleswig in Danish.
Lack of Prussia forced Brits to find new ally in Europe to counterbalance Russia (Hannover is sword against France), ideal candidates are Austria and... Poland-Lithuania, there is only one problem-russian domination in Poland, but this probleam is solvable-after revolutionary wars army of PLC was rebuild, With secret aid of british intelligence Poles started uprising around 1830, something like enlarged November uprising with support of Austria and Saxony (OTL Russia had problem with uprising started by tiny Congress Kingdom, ATL they faced several times larger PLC, plus Saxony and Austria (and british diplomatic support) as result Poland-Lituania is no longer in russian sphere. It is how system of alliance in Europe looks now:
-Austria, PLC-Saxony and Hannover are british allies against France (Hannover, Austria) and Russia (PLC-Saxony, Austria) PLC is good ally for Austria, 17th century showed it.
-France and Russia become allies (both of them have interests against London)
Now German unification is totally screwed:
-Hannover in personal union with Britain, Saxony with PLC-so PLC and Britain against unification.
-France, despite being in another camp is natural enemy of unification especially if led by Austria, but I think they would try back Bavaria agains Habsburgs.
-Sweden and Danemar for obvious reason are against too.
-Austria is unable to unite German-is too much concentrated in Balkan/Hungarian region, but is able to stap attemps of unification by other states.
I think it is perfect situation to keep Germany disunited, almost everyone have interest to prevent it including the most important players of Europe.
German revolutionaries have to deal with Britain, France, PLC, Austria, Sweden and Danemark, not to mention conservative forces inside German states.