I doubt if the Russians would want Posen - they have enough trouble holding down the Poles they already have.
The rump Prussia is probably Brandenburg (perhaps minus the Altmark) Pomerania, East/West Prussia and Posen. It will be left out of the German Confederation (which it had denounced at the outbreak of war) with a ban on readmission for at least 20 years, by which time Austria will have the GC reorgansised its way.
Austria takes Silesia, in return for writing of Venetia, Lombardy, and the GDs of Tuscany and Modena. The bulk of the Rhine Provinces gets carved up into new principalities for the two GDs, the balance (plus maybe other Prussian territory west of the Elbe) gets shared out between Bavaria, Wurttemburg, Hanover and Saxony, the latter also getting those lands east of the Elbe which it lost in 1814. Schleswig-Holstein goes to the Duke of Augustenburg, with N Schleswig possibly going back to Denmark.
To appease France, the former Habsburg possessions in Italy are written off (see above), though a defeated Italy would be made to "buy" them for several times their value. France is also left free to annex Luxemburg if it wants to.
The newly installed Hohenzollern Prince of Rumania is probably forced to abdicate. The successor can't be a Habsburg (unacceptable to Russia) or a Romanov (ditto to Austria) so is probably from some minor German house.
Russia takes the opportunity to denounce the Black Sea clauses of the peace of 1856, as she would do OTL in 1870.
Prussia has to pay a heavy indemnity, to be held by Austria in trust for the GC. Any other German states looking for payment of their war expenses must come cap in hand to Franz Josef for them.
The minor German states which sided with Prussia are put under Federal occupation pending a final settlement. Princes who fought against the GC must abdicate or renounce rights of succession, but Franz Josef, as President of the GC, may set aside this penalty. IOW they are totally dependent on his goodwill and must vote his way in the Diet.