During his reign over the Rashidun Caliphate, Caliph Umar sent a delegation to the Chinese Emperor (Tang dynasty at this point in time). While I would assume they discussed other things as well, the delegation tried to convince the Emperor to convert to Islam (he didn't, but funded the first Mosque built in China).
So, something I've been thinking about recently. What would/could have happened if this delegation had been successful in this area, and the Emperor had become a Muslim?
I don't know much about Chinese history pre-1800s, but I'll give it a shot.
It would be hard for the Emperor to convert to Islam, seeing that it would be something like the apostle Paul trying to convert Nero. However, assuming that such a thing does happen, there are two things that could happen from there:
1) Palace officials and other members of the royalty disapprove the move and move to remove the Gaozong Emperor. China's history may be altered somewhat significantly because of this, but society still remains somewhat Sinicised with with much of society centered around Buddhism, Taoism and Confucianism. Muslim Han Chinese and the Uighur would still be a minority in China.
2) Massive butterflies occur as a large part of China converts to Islam. Given the entrenched roots of the Chinese culture, I would expect the Chinese to attempt to somewhat assimilate Islam into their own society instead of the other way around. Perhaps claims that Confucius was a messenger from Allah? Buddhism probably becomes suppressed and this may fragment China along religious lines. It may also cause a rift in Islam as the Chinese practice a "deviant form" of Islam, given that the Chinese will probably assimilate the new religion into society. Chances are, this may have a potential spillover effect on Korea and Japan and perhaps Indochina and the rest of Southeast Asia.