So, when will this TL start?
Maybe after some other kinks are ironed out. I think Prussia can keep Neumark, but she'll lose Hinterpommern.
Who will Britain ally with since Prussia is no longer viable?
A Russia estranged from her allies?
An Austrian Empire feeling once again threatened by the French?
If Austria picks up all those little Prussian peices out west, that would be a pretty contigious block between Hannover and France. I'm not sure wether or not Britain would take this as a good thing or wether or not thats plausible.
With the loss of Prussia proper, does Prussia still call herself that or is it to be called Brandenburg from now on?
I'm thinking that Catherine has some harder troubles knocking her husband off the throne and she gets sent to a nunnery under guard, and some of her conspirators are executed. This leads Peter III not to support Sigismund II Augustus' candidacy, as he sees him as too close to Catherine (ex lover and all).
This lets Charles Edward Stuart press his candidacy with French backing (a POD could be he does not show up drunk to the war meeting, and to prepare for the (ultimately doomed) planned invasion of England, he keeps sobering up giving the French a better opinion of him) and he marries Maria Amalia of Austria to cement his ties with them. He could also ally himself with the progressive party.
Empress Elizabeth lives for that extra year (the illness sets in late, or she secretly takes the medicine), but it soon becomes very clear to everyone what a Prussophile her heir is, so the anti-Prussian coalition are quick to make a deal with Britain (who gives back Guadeloupe and Martinique and loses Minorca) so they can finish off Prussia.
Prussia gets disembered. Brandenburg and Neumark stay together, but loses Magdeburg to Saxony, Silesia to Austria, East Prussia to Poland (who loses a chunk of Eastern Lithuania to Russia), and East Pomerania to Sweden. I'm not sure what would happen to Hinterpommern, but I think it would get shorn off as well. Wether that it be given to a Bourbon, a Wettin, a Romanov, or whoever I'm not sure of yet.
Frederick the Great of course is never known as the Great, and commits suicide. Thus the entire concept of partitioning Poland never comes up, and Catherine just continues to see Poland as her little playground.
I'm thinking that Great Britain will keep most of her gains, but I'm not too sure about Florida, though I'm thinking along the lines that the whole new World will turn out pretty much like it did in OTL after the war ended.
This of course will probably lead up to a American War of Independence, but if that was to happen, I would make sure it goes very differently.
Eventualy of course, Catherine the Great will overcome her husband, just a year or too later than OTL.
Again, I still need to think about it and work out some unknowns.