During the Arab Revolt, Mecca was lost to the Arabs fairly early in 1916. As the holiest city in Islam, you would think that the religious legitimacy of the Ottoman sultan as Caliph would depend on them maintaining control of the city. However, it seems that the Ottoman plans for late in the war and post-war took a more pan-Turkic route rather than a pan-Islamic one, especially after the capitulation of Russia. Was there a plan in place to re-establish Ottoman control over cities like Mecca, and to a lesser extent Jerusalem and Baghdad, or were they considered essentially lost to the British and the Arabs?
The short answer is: no. The First World War de jure concluded between the Ottomans and the entente when they signed the Armistice of Mudros in 1918. The peace treaty that they signed was the 1920 Treaty of Sevres, which would have reduced Ottoman territory to a rump state around Ankara from which they could not possibly hoped to have overcome the surrounding powers by force.
The 1920 treaty was rejected by Turkish nationalists, led by Ataturk, who in one of the last acts of the Ottoman parliament declared the Misak-i-Milli, the National Pact. The Pact explicitly declared that the Turks would make no claims to former Ottoman possessions in Arab-majority territories (the status of those territories could be decided by a referendum), but territories not occupied at the time of the signing of the armistice in 1918 constituted an indivisible Turkish homeland. This was the strategic/ideological claim of Ataturk and the Turkish nationalists that carried them through the series of wars that eventually resulted in Turkish independence within almost identical borders to the ones it currently holds as a result of the signing of the Treaty of Lausanne and the declaration of the Republic of Turkey, both in 1923.
Initially, Ataturk claimed that intended to preserve the Caliphate, even though the Ottoman Sultanate had been dissolved in 1922. He eventually dissolved the Caliphate as well in 1924. It's not entirely clear how seriously he ever intended to preserve the Caliphate, or if it was merely a claim made in the strategic interest of creating a united front during the Turkish independence wars.
you would think that the religious legitimacy of the Ottoman sultan as Caliph would depend on them maintaining control of the city.
That was historically not all that important, largely because the Caliphate itself by this period was not all that important (thus it was abolished without too much fuss). But it was the basis by which Sharif Hussein of Mecca tried to declare himself Caliph after the war. That claim was not widely recognized and he was eventually forced to abdicate during the Saudi conquest of the Hejaz, with his sons declining to try to take up the title.
