Now then, interesting premise. After the Burgfrieden era is over, the German society will continue to polarize. The old Junkers-led state apparatus will remain intact, but compromises with Noske-type moderates are increasingly necessary as left-and right-wing political extremism starts to rise. Paramilitary organizations and veteran services are bound to rise due the shared experiences in the trenches but disagreements on the future direction of society. Election system reforms in Prussia are a must, and will further degrade the position of old elite. They might start to seek for compromise candidates to lift their own support.
In my own TL a similar situation leads to a rise to a bit different but still recognizable JungDo, with Volksnationale Reichsvereinigung remaining independent political force and organizing closer collaboration with Deutsche Vaterlandspartei-type umbrella organizations the establishment sets up as populistic supporters of their continued rule. The left will meet this challenge by reorganizing, and some kind of radical wing is bound to split from Social Democrats sooner or later. What kind of political direction and agenda they'll have depends largely on Russia, though - Menshevik and SR example of constitutional Soviet power instead of Leninist terror tactics will create huge butterflies.
I doubt A-H will fall apart. They limped through the war almost to the end, and with the KuK Armee intact, the leaders of various nationalistic movements will be too busy distancing themselves from their war-era secrets contacts with their discredited pro-Entente powers to plan anything serious. Especially because becoming part of German Empire is a worse solution. Inclusion of Poland might well be acoompanied by trialism, with Poles of Galizia and Poland proper acting as a counterweight to Hungarian ambitions. Nationalistically Poles and Hungarians have always had warm relations, while Poles would look Vienna as a necessary protector against Berlin and Moscow. It's not going to be all happiness and sunshine, but the Dual Monarchy was resilient and disintegrated only because they utterly lost the war in OTL. They'd be messy and troubled, but would move along and start to invest on their huge economic potential as the new heartland of the new Mitteleuropa customs area.
As for other powers - it is too late to reverse the Ottoman course towards Pan-Islamism and Ottomanism - they will still see themselves as primarily Turkic empire, but new Arabs revolts are unlikely. Oil revenues will start flowing in the future, and the Empire will remain a poor and internally troubled German client to the foreseeable future.
Poor Italy, they fought a lot and lost. Instead of revanchism this might actually ultimately lead to moderate coalitions who cut military spending and accept the status quo, while left-and right wing forces continue to fight among themselves.
Britain will mostly focus on the issue of Ireland in the near future, and reorient their foreign policy to accept the fact that City is no longer the financial hub or the world and that Germany is the new continental hegemon.
Poor France. Her domestic politics will remain as a turbulent mess to foreseeable future, but I doubt that outright revolution would break out.
Russia is interesting. With Lenin still safely rotting away with his brain syphilis at Switzerland, the left-wing forces will find a common ground and the Constituent Assembly elections will take place. In OTL it took Lenin a lot to get the Bolsheviks strong enough even in the Petrograd Soviet - let alone to speak his comrades away from the idea of cooperation with Mensheviks, SRs and other leftists...Bolsheviks were losing their early wave of ethusiasm in the local soviet elections in late 1918 in OTL as well, and that was one of the main reasons why Lenin opted to stage a coup in the first place - electional defeat and a new left-wing coalition government loomed on the horizon.
Without Lenin to start his Red-Green internal struggle, the conservative elements in Russia are too weak and dispersed to oppose the main goals of the Mensheviks and SRs - land reform and new Soviet model of government. Both reforms are bound to cause wide unrest and problems in the former empire, combined with separatist movements in the fringes of the Empire. Finland will most likely regain her pre-Russification autonomy, and the Finns will generally be happy enough with that when they compare their lot to Estonians, who emerge from the Russian yoke only to find themselves bossed around with the same old Baltic Germans who are now backed up their Prussian cousins.
All in all, troubled and turbulent 1920s lay ahead. Economically and politically it is pivotally important for Germany to invest heavily on her troubled former allies to keep them from falling apart, and to create new markets to her products.