Let us assume the war ends in 1918 by a successful Spring Offensive. France is knocked out of the war. It is not clear why the UK and US would also want a peace when the shadow of the future was on their side. But let us assume for whatever reason the US, UK and Germany decide to ram a peace through. By default this will be a negotiated peace as UK-US power is too much for Germany to impose anything unliterally on them. So peace largely in the state of the fronts in August 1918. What does that mean on the Frontlines
Middle East-No Battle of Megiddo, so no collapse of Ottoman Positions in Palestine, Bagdad in British hands.
Caucasus- Treaty of Batum in power, Transcaucasian Federation broken, and Ottoman control of Batumi and Baku, simmering Ottoman-German conflict over Georgia
Italy-2nd Battle of Piave stabilizes Italian front, but no Vittorio Veneto
Balkans-Skra still happens leading to change of Goverment in Bulgaria but no Dobro Pole
This largely gives a potential idea of how the German's get peace from the US and UK. A neutralized France (with a regime change), and avoidance of major territorial changes in the West (instead opting for economic integration) and recognition of German gains in Eastern Europe and the Balkans in return for recognition of British gains elsewhere, mainly the Middle East.
For the Ottomans this means the following: 1) No gain of Egypt or Cyprus 2) Loss of Mesopotamia and what today is Israel and Palestine, but retention of Mosul and Syria. 3) I fully expect the Germans to press them hard on the Caucasus were they already had fought each other . And I expect the Germans to use the British for this. Thus Britain gets Iran in order to cooperate in forcing the Ottomans to evacuate Baku. The Azerbaijani Republic becomes a Ottoman-German condomium (ala Iran as a Russo-British one), Rump Armenia is either made a Ottoman protectorate or included in Azerbaijan as an autonomous region to maximize intervention opportunities , while Georgia becomes a German Protectorate. The Germans probably let the Ottomans have Batum, but with major basing and commercial concessions. Another area for compensation for the Ottomans would be granting them back the Eastern Aegean Islands taking them for Greece that probably has fallen into a deep political crisis.
Now the good news for the Ottomans is that they keep a lot of land they might had lost, and gain territory but they now border at least one Arab state, with a province (Syria)that was an epicenter of arabic demands for political equality and autonomy, and are in active competition with Germany in the Caucasus. Also that region is now also the main area of habituate of the Ottoman Armenian population. There is no 1918 re-patriation of Ottoman Armenians so the bulk of the expellees are now stuck in Syria. Thus the Ottomans border on two shatterzoes (Caucasus and Syria-Mosul) with hostile major powers able to influence local populations. Thus at least in those two areas they do face a threat. On the other hand no collapse of 1918 means amelioration of the nationalities issues. The Armenians are broken by genocidal policies, and scattered among Kurdish and Arab populations in Syria, or sandwiched between Azerbaijan and the Ottoman Empire. About 1/3rd of Greeks were expelled or deported in 1914-1918 . As with Ottoman Armenians they are not going to be repatriated as in historical 1918. I expect an intensification of the types of population engineering Fuat Dundar noted, and a continuation of the push to force the majority of the Ottoman Greek population to involuntarily migrate.
So a lot depends on a) how well the Ottomans balance Britain and Germany b) how well they accommodate Syrian Arab demands for autonomy, and perhaps Kurdish ones c) how they leverage Soviet renaissance . My view is that they will fail, and thus long-term I do see them losing Syria, and potentially Mosul. But they probably keep most of their eastern and western borders.
For the Austro-Hungarians. Well you already had a looming constitutional showdown before 1914 and I cannot see how this is avoided after victory. Probably the Austro-Hungarians are forced to vacate most of Italy with he exceptions of key military sites and border adjustments. The Serbian problem is resolved by letting Bulgaria keep what it got, attaching Kosovo to Albania, and establishing a Montenegrin dynasty in Serbia but prohibiting a union of Montenegro with Serbia. This will require though a long term military occupation and you can expect bickering between Vienna and Budapest over control. Both the Austrian and the Hungarian parts will face a constitutional crisis but it is hard to predict how it turns out. But unlike in the case of the Ottomans, I do not expect any German reason to make their life difficult, and Britain will probably stay out of this just keeping preeminent influence in a smaller Greece as part of a network of positions permitting it domination of the East Mediterranean.I do expect the Germans to seek to domiante Romania the same way they will domiante Georgia, giving them if you want two opposite shores for domination of the Black Sea.
So I expect the Austro-Hungarians to go through some rough political times but no dissolution.
So long story short. Things are more difficult for the Ottoman due to a) competition with Germany b) presence of a powerful UK, but less difficult for he Austro-Hungarians.
