Steffan, outside of a general European War there is zero chance of Britain grabbing any Ottoman territory, with the possible exception of a Russian attack upon the Ottoman Empire on a scale that causes it to fall, which is a dubious prospect at best. In the entire history of the Ottoman Empire, the British did not once grab an integral part of the empire other than WWI, and the "lease" of Cyprus after the 1877-78 War, which was negotiated with the Ottomans and was intended specifically to provide them with a base to defend them against further Russian incursion. And, the terms of the treaty, which remained in effect until WWI, the Ottomans retained control over all the domestic administration, and all revenue surplus to paying for the British presence was remitted to the Ottoman treasury (or toward the Ottoman debt).
Egypt, which was largely independent, was occupied by Britain in 1882 against the will of the British government, which had wanted the Ottomans to occupy it with British assistance, and for several years afterwards the British tried to get the Ottomans to assume control.
France was allowed to occupy Tunis by the British as a quid pro quo for accepting British control over Cyprus, and Italy was allowed its free hand in Libya for largely the same reason and had to spend decades preparing the diplomatic grounds for this.
So, that leaves us with the military dimension. Which powers were capable of defeating the Ottomans 1 on 1? Here is a list of possible powers:
Italy
France
Russia
Greece
Britain
Italy wasn't even able to defeat the Ottomans in Libya; on the mainland, they would have a zero chance of beating the Ottomans in an invasion.
Ditto France.
Greece was crushed by the Ottomans in a couple of weeks when Greece invaded in 1897; in 1922 they lasted longer, but the Ottoman army had been demobilized and Anatolia was being simultaneously invaded by Armenia, Greece, France, Italy, Britain, and Russia.
Britain could probably muster the military strength to win, but I would say would have no chance to generate the casus belli or public opinion to do so, nor would the possible rewards be worth the effort - after all, what is to be gained? They have economic interests, which are already settled, and the Suez Canal, which is already under control.
That leaves Russia. A seaborne invasion has no chance. Yes, they planned it, but where Britain and France failed miserably, Russia will not do better. The only other route of invasion is through the Caucasus. In WWI, while the Russians were invading, the Ottomans were having to deal with Gallipoli, Palestine, and Mesopotamia. Nevertheless, the Russians never got more than a hundred or so miles across the border, largely due to the failure of an insane winter offensive launched by Enver. In order to plow across the mountainous Anatolia, through the Ottoman heartland, Russia would have to expend enormous resources, and I'm not sure Russia had the stability to pursue an offensive war at this time for gains that would not be important to the Russian populace.
Also, by the 20th c, the tables were turning; the Ottoman Empire after the Balkan Wars had become religiously homogeneous and no longer had any territory containing a seperatist majority, whereas the Russian Empire was suffering from severe instability. The Ottoman army was in the process of totally restructuring when WWI began, so the window of opportunity for Russia would have been very small, and after the Balkan Wars, its hard to imagine any source of casus belli.
The Orientalist view of the Ottoman Empire as some feudal state that reached its peak under Suleyman and went steadily downhill after that is a relic of the 18th century that dies hard. First of all, the Ottoman Empire was never a feudal state, and it continuously evolved over time, it didn't freeze in place in 1550. In the middle of the 19th c it began a furious program of reform that by WWI had produced a relatively modern polity with a secularist outlook and a representative parliament. Modern Arab nationalist states tend to downplay the Ottoman period as one of stagnation and oppression, but Syria, Arabia and Iraq were very much an integral part of the Ottoman polity, not occupied colonies; a large portion of the army defending Gallipoli was Arab, as was a large part of the Ottoman ruling class, including the Grand Vizier. The "Arab Revolt" consisted of a few thousand beduin tribesmen, the vast majority of the empire's Arabs (and Kurds) remaing loyal to the end, even though promised independence (they were not naive about this). In case of a naked imperialist grab by Italy, their defense would be furious.
As to the overall powers of resistance of the Ottomans in the WWI period, you need look only at WWI. After losing half the army in the Balkan Wars (due to a stupid strategic plan and an inexplicable decision to reduce the standing army by 1/3 just prior, plus the fact that the army was undergoing a massive restructure at the time and most of the best officers were in Libya resisting the Italians), the Ottomans were still able to fight on seven fronts simultaneously and manage to hold on more or less intact until 1918, even then conquering the entire trans-Caucasus, and never enduring any civil disturbance in the process, then going on to defeat all comers in the invasions after the war, after having demobilized - that's 12 years of total war against and enormous array of enemies. That's pretty impressive, given the resources of the empire. And those resources would only have increased as time passed, given the oil.
With regard to Slavic nationalism, all I can say is, whaaa? Serbian ambitions for a Greater Serbia began in 1830 when they gained autonomy, and they had been feverishly eying Bosnia since that time. This did not begin with WWI; if anything Serbian ambition CAUSED WWI. Actually, my principle problem with this WI is that Serbian ambitions would eventually have led to a showdown with Austria-Hungary, and the alliance system would still have done its thing. AH specifically occupied BH to forestall its falling into the hands of Serbia.