The challenge: make Tibet convert to Islam instead of Buddhism. POD can be any time after 622, however you must not change anything fundamental about the Islamic religion or Middle Eastern politics (minor changes are allowed, but there still has to be Ummayyad and Abbasid caliphates and all).
Also, what might be some of the effects? In particular, I think it would be interesting to see a Muslim power, probably an expansionistic one (I understand Tibetans were expansionistic before they became Buddhist, and Islam doesn't stress pacifism nearly as much as Buddhism does) near China (doubt it would be strong enough to do much while China was powerful, but during interdynastic periods it might be interesting).