As im sure most of us know, after the Arab revolt in World War 1, which was a Hashemite lead effort, Feisal al-Hashemi tried to build up support amongst the British in paticular for a Arab state encompassing the Arab lands of the former Ottoman empire. However, the British decided that allowing their French allies to have their spoils of war was more important then rewarding Feisal's efforts during the war. The French occupied Syria and defeated Fesial when he tried to resist militarily.
So the challenge is, to give Feisal control of the Levent by 1930, with any POD from January 1st 1915. Bonus points if this Greater Syrian state has Mesopotamia and is not subordinate to Britain in some way (such as Mandate, ect).
Nassirisimo
The obvious might be that Gallipoli works but hard liners fight on from the interior of Anatolia. This is fairly trivial for the allies as they are isolated and the allies have control of the straits and a secure position in the Balkans as Greece and probably Bulgaria will join the allies. However it does mean that the continued opposition causes problems and possibly Townsend still fouls up in Mesopotamia. Hence the British see a need to support the Arabs, who are encouraged by the serious defeats the Ottomans have suffered. Coupled with anger at the Armenian massacres opinion is that the Turks must to punished so the desire is to remove their Arabic lands. Against a weaker opposition British & Arab forces occupy most of the region as of OTL 1918 but by say 1917 when this war ends.
Because of the improved position in the east Russia doesn't fall and France is in a better condition. Hence the powers are able to agree a more stable peace in Europe. France wants some say in Syria, where it historically had interests but with pro-British Arabs already in place this is traded away for something somewhere else.
I can't see the new Arab state not being under a British 'Protectorate' at least unofficially if not formally. Say the sort of situation as with Egypt before the war but with less British open presence because it doesn't have the same significance - at the time anyway.
This would also have the advantage that the Hashemites would then have a more powerful position militarily and in terms of prestige and almost certainly get British support, if it was needed, to keep the Saudis in control. It might not be a single unified state, if only because of the size but you might have a serious of Hashemite states, with one, say Syria, being the senior line.
Other than keeping order and the discovery of oil in the east the area would be largely unimportant as far as the wider world was concerned so it would probably have increasing independence and with say a government in Britain cutting back on commitments, having both de-facto and de-juro independence by 1930.
You might get a similar path with an unsuccessful Gallipoli but that does make for a longer and more bitter war and gives more time for France to want to get involved. Also if the war is that much closer and especially if Russia falls into the Bolshevik coup, the situation in Europe is that much more unstable so Britain needs France more.
Anyway, my initial thoughts, for what their worth.
Steve