I don't know why everyone is talking about 1918 victory, when the OP clearly states the war is over by 1915.
Let's say literally everything goes right for the CP. Like so:
- Conrad falls out of favor after the annexation of Bosnia, as shortcomings of the army are highlighted despite the quick victory.
- New Chief of Staff implements some reforms and the war plans with Russia are scrapped for a more defensive one in Galicia, with the overall plan focusing on Russia before Serbia
- Early 1914 fight do not see the heavy losses on the AH side as OTL, but Russian headlong attacks into Galicia are more crippling
- Schlieffen goes as planned, Miracle of the Marne does not happen, Paris is either taken or comes in range of German heavy guns and rendered useless as a logistic hub
- Race to the sea won by Germany as no forces are transferred to the East
- Tannenberg happens as OTL, Russian army is completely thrashed on both North and South
- Western Front devolves into trench warfare, French counterattack to dislodge the Germans from Paris fails, France collapses
- Britain retreats from the continent, but stays in the fight, trying to enforce a blockade
- Bulgaria joins the CP, opens new front in Serbia (a very silent front with minor skirmishes so far at best)
- Early 1915 an ATL Gorlice-Tarnów happens, unraveling Russian lines, Russia informs Britain they will sue for peace
- German and AH forces transfer forces to Serbia, joint German-AH-Bulgarian offensive takes place over the summer, Serbia collapses, government cannot escape, capitulates
- Romania joins the CP
- Russia signs an armistice as there is no political will after Serbia's fall
- Britain also comes to the table, war officially over by September 1915
Under these conditions we are looking at a very different Europe, where the idea of minor nation states is not exactly feasible. Any nationalist elements in AH would push for reform rather than independence, and it might drift towards a Federal system over the next few decades, but definitely no collapse. The Habsburgs reign supreme, Franz Josef is a great victor, Karl takes the throne in a much less tumultuous time. The common struggle of the various nationalities in the army during the war is probably highly mythicized and used for propaganda to rally them around the flag. In short, the old order prevails, small nation states are not a very inviting possibility in a Europe still ruled by huge empires.