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Historically, the Hijaz (the green area on the map) was the last part of modern-day Saudi Arab to be incorporated into the Kingdom. Traditionally, the area was rulled by the Hashimite House, and one of the Hashimites (IIRC, Hussain) was proclaimed "King of the Hijaz" after World War I, but the Hashimite's rivials, the Sauds, invaded and annexed the area in the early 1920's.
So, what if a Hijazi state had survived-say, for some reason, Britain's relationship with the Sauds becomes worse and they intervine to protect their Hashimite allies? A Saudi state would probably form in the rest of its OTL territory, and it would still have its oil (which is mainly located in the east). However, it wouldn't control Mecca and Medina, and thus would be denied the prestige benefits that entails. Unlike the Sauds, the Hashimites were not traditionally Wahhabi, so I'm wondering if we'd see an anti-Wahhabi theological establishement grow up in Mecca under Hashimite patronage. This might lessen the influence of Wahhabism outside the Arabian penninsula (thought its main strength, Saudi oil money, will be unaffected).