The next great European conflict between two large rival factions breaks out in the 1850s instead of the 1910s.
Vienna likely doesn't sway many, or even any, of the German states to join her in this war. Prussia is definitely going to sit on the sidelines; however the war might bring about German unification quite a bit earlier, as Bismarck was the Prussian Ambassador to the German Confederation at the time, and with Austria distracted by war with the Entente allies, and the Zollverein in full-swing, Berlin may try to resurrect the Erfurt Union, or something similar. Certainly a proto-North German Confederation, which would be a more federal, and equal, framework under Prussian leadership rather than out-right Prussian domination and annexation as per OTL is certainly in the cards.
Expect this to be a repeat of 1848 in many ways. The Hungarians are very likely to take another chance to rise up, as are the Italians (an earlier Campagne d'Italie is very likely). This however also means that the Croats, Serbs, Montenegrins, and Romanians are all likely to rise up as well, and for the latter groups their nationalistic revolutions will be more complicated in their loyalties towards both the Entente and the Imperialists.
Importantly considering the alliances and states involved in the war the Piedmontese aren't likely to join the war from the get-go with Austria involved, as Cavour was opposed to the war even IOTL and will have more sway in court if there's a threat of having to face-down with Vienna again without strong allies (France will, understandably, be more interested in fighting in the Crimean than she will in the North Italian plain) and will only be dragged into it after the Italian radicals start to rise up once again in the Central Italian states and Venetia-Lombardy; just as in 1848. This means that the Savoyards likely won't secure their OTL position as the main focal point of Italian unification. This is especially important due to the timing - the Garibaldian, Mazzinian, and Guerrazzian factions of the radical Italian nationalistic republicans have yet to have their falling out, and will be as united, or even more so, as they were in 1848.
Expect things to take on a truly world war-esque scope if this conflict drags out even longer than the Crimean War did IOTL, and the Anglo-Persian or Second Opium wars, or the Indian Rebellion, happen on schedule as per OTL.