For purposes of the discussion, we'll assume the following:
1. Ottomans remain neutral (and are allowed to stay that way, with stuff like Britain not invading Mesopotamia or a parallel Greco-Turkish war erupting)
2. The war drags on for ~4 years, give or take, with everyone exhausted and Germany defeated
3. Russia has collapsed into a fractured civil war, much like the one IOTL
4. Bulgaria was part of the CP
Off the bat, the main problem with this is that Ottomans staying neutral greatly helps the Entente, meaning the war may very well end sooner. Therefor, let's add some factors that bring back balance. These are by no means perfect, and you may very well substitute your own, but their point is to help out the CP war effort:
1. Galicia '14 ends up as an A-H victory instead of the unmitigated disaster of OTL
2. Italy stays neutral for a wee bit more, say a year, lifting some pressure off the Austrians
3. There is no Brusilov offensive and reorganization of the army. Instead, things go from bad to worse for the Russians, leadership wise.
4. As a consequence of #3, Romania never enters the war except at its very end, when the CPs are already collapsing anyway
5. There is another colonial theater to distract the Entente somewhat - perhaps the pro-German coup in China works. Who knows.
Thus, by the end of 1918, the world is in a similar situation to OTL, except that the Ottomans had stayed neutral through all of it. It's maybe not the most likely of outcomes, but it's by no means ASB either.
So, what now? Perhaps ITTL the Ottomans did a move similar to Romania in 1918 (as well as in our alternate 1918) or to Turkey in 1945 OTL, and declare war on Germany & friends a couple of days before the latter surrender anyway. What can we expect them to gain out of all of it?
For a start, it's nigh-certain that the economic concessions Constantinople had granted to Germany, Austria and Russia are gone. Maybe or maybe not the did the same to the ones offered to Britain & France at a moment where these seemed weak. The massive loss of life experienced by them is averted, including the Armenian Genocide. The Arabs never revolt in the first place. The railway network may have been strengthened throughout, but is by no means complete.
Next, territory.
What I think is possible is that the rather loose control over Arabia is strengthened ITTL. The Ottoman vassals of the Emirate of Jabal Shammar have been fully incorporated at a minimum, with regular Ottoman army outposts dotted across the desert. Ryadh may well have been conquered by an Ottoman-Rashidi force ITTL from the Saudis whilst the British were to busy to care. What happens to Kuwait is uncertain, though Turkish influence would be much increased if there is now a rail line running all the way from Baghdad to Basra
To the north, the Caucasus is almost free real-estate. Azerbaijan was effortlessly taken from the Russians at the tail-end of the war anyway - IOTL, most of the rest of the Russian Caucasus is occupied once the large professional army Ottoman army, who has had 4 years to build up supplies near the border, marches in, and there's hardly anything apart from disorganized militias there to oppose them. Chechnya and Dagestan, with their Muslim majorities, are probably occupied as well (with the Ottomans even being viewed favorably by the locals), and depending on just how deep the Russian disorganization runs, parts of Circassia maybe as well. With the latter having been the site of a Russian genocide (https://en.wikipedia.org/wiki/Circassian_genocide) only a couple of decades prior, many Circassian refugees which had been welcomed into Ottoman lands may try to return there. Lastly, there is Crimea, which at the time was IIRC about a third Tatar/Muslim, a third Russian and a third others. Whether or not the Ottomans try to make an attempt to retake it, idk.
To the east, we have Iran, which at the time had a Russian sphere of influence verging on outright occupation in the north (and an equivalent British sphere in the south). With the collapse of Russia, the Ottomans may attempt to flex their muscles somewhat in this area (which logistically was not beyond their reach, given that they briefly managed to occupy Tabriz twice OTL during the war. Further east, there was Russian central Asia, which OTL saw significant Turkmen opposition to the Soviets, in which none other than Enver Pasha was involved. Once they seize Baku, I think it's a near certainty that the Ottomans will become involved in supporting these Muslim uprisings, meaning that by the mid 20s we might be getting some intense naval action in the Caspian, as the Russian state (whether White or Bolshevik) attempts to cut these efforts off.
Lastly, in the west, the Ottomans will stake their claim to Eastern Thrace bits of eastern Bulgaria at the peace conference where there were substantial Turkish populations, or maybe even a return to 1912 borders. Whilst the former are possibly granted by the Entente, the latter are probably rejected. Finally, there are the Dodecanese, which the Italians were nominally obliged to return according to the Treaty of Ouchy, a fact which the Balkan wars and then WW1 prevented. Here, they may run out of excuses, although Ottoman attempts to occupy them may very well lead to a war with Greece
1. Ottomans remain neutral (and are allowed to stay that way, with stuff like Britain not invading Mesopotamia or a parallel Greco-Turkish war erupting)
2. The war drags on for ~4 years, give or take, with everyone exhausted and Germany defeated
3. Russia has collapsed into a fractured civil war, much like the one IOTL
4. Bulgaria was part of the CP
Off the bat, the main problem with this is that Ottomans staying neutral greatly helps the Entente, meaning the war may very well end sooner. Therefor, let's add some factors that bring back balance. These are by no means perfect, and you may very well substitute your own, but their point is to help out the CP war effort:
1. Galicia '14 ends up as an A-H victory instead of the unmitigated disaster of OTL
2. Italy stays neutral for a wee bit more, say a year, lifting some pressure off the Austrians
3. There is no Brusilov offensive and reorganization of the army. Instead, things go from bad to worse for the Russians, leadership wise.
4. As a consequence of #3, Romania never enters the war except at its very end, when the CPs are already collapsing anyway
5. There is another colonial theater to distract the Entente somewhat - perhaps the pro-German coup in China works. Who knows.
Thus, by the end of 1918, the world is in a similar situation to OTL, except that the Ottomans had stayed neutral through all of it. It's maybe not the most likely of outcomes, but it's by no means ASB either.
So, what now? Perhaps ITTL the Ottomans did a move similar to Romania in 1918 (as well as in our alternate 1918) or to Turkey in 1945 OTL, and declare war on Germany & friends a couple of days before the latter surrender anyway. What can we expect them to gain out of all of it?
For a start, it's nigh-certain that the economic concessions Constantinople had granted to Germany, Austria and Russia are gone. Maybe or maybe not the did the same to the ones offered to Britain & France at a moment where these seemed weak. The massive loss of life experienced by them is averted, including the Armenian Genocide. The Arabs never revolt in the first place. The railway network may have been strengthened throughout, but is by no means complete.
Next, territory.
What I think is possible is that the rather loose control over Arabia is strengthened ITTL. The Ottoman vassals of the Emirate of Jabal Shammar have been fully incorporated at a minimum, with regular Ottoman army outposts dotted across the desert. Ryadh may well have been conquered by an Ottoman-Rashidi force ITTL from the Saudis whilst the British were to busy to care. What happens to Kuwait is uncertain, though Turkish influence would be much increased if there is now a rail line running all the way from Baghdad to Basra
To the north, the Caucasus is almost free real-estate. Azerbaijan was effortlessly taken from the Russians at the tail-end of the war anyway - IOTL, most of the rest of the Russian Caucasus is occupied once the large professional army Ottoman army, who has had 4 years to build up supplies near the border, marches in, and there's hardly anything apart from disorganized militias there to oppose them. Chechnya and Dagestan, with their Muslim majorities, are probably occupied as well (with the Ottomans even being viewed favorably by the locals), and depending on just how deep the Russian disorganization runs, parts of Circassia maybe as well. With the latter having been the site of a Russian genocide (https://en.wikipedia.org/wiki/Circassian_genocide) only a couple of decades prior, many Circassian refugees which had been welcomed into Ottoman lands may try to return there. Lastly, there is Crimea, which at the time was IIRC about a third Tatar/Muslim, a third Russian and a third others. Whether or not the Ottomans try to make an attempt to retake it, idk.
To the east, we have Iran, which at the time had a Russian sphere of influence verging on outright occupation in the north (and an equivalent British sphere in the south). With the collapse of Russia, the Ottomans may attempt to flex their muscles somewhat in this area (which logistically was not beyond their reach, given that they briefly managed to occupy Tabriz twice OTL during the war. Further east, there was Russian central Asia, which OTL saw significant Turkmen opposition to the Soviets, in which none other than Enver Pasha was involved. Once they seize Baku, I think it's a near certainty that the Ottomans will become involved in supporting these Muslim uprisings, meaning that by the mid 20s we might be getting some intense naval action in the Caspian, as the Russian state (whether White or Bolshevik) attempts to cut these efforts off.
Lastly, in the west, the Ottomans will stake their claim to Eastern Thrace bits of eastern Bulgaria at the peace conference where there were substantial Turkish populations, or maybe even a return to 1912 borders. Whilst the former are possibly granted by the Entente, the latter are probably rejected. Finally, there are the Dodecanese, which the Italians were nominally obliged to return according to the Treaty of Ouchy, a fact which the Balkan wars and then WW1 prevented. Here, they may run out of excuses, although Ottoman attempts to occupy them may very well lead to a war with Greece
