Japan would be Something Interesting depending how unfold(even someone mentioned going Sunni just to diverge from China) yeah whatever that archipelago ended up, will be significant for the tl, NOT much unlike MECA(Middle East and Central Asia) but Important
Yeah, here's a short points on Japan-China relationship pre-Meiji.
1. Pure Sinophilia in the Asuka and early Heian Era, with T'ang Dynasty culture and artefacts and religion adopted wholesale, including Buddhism, Kanji, the Imperial Court, and the Samurai "aristocratic gentlemen of war". However, Japan doesn't exactly put themselves as Chinese tributary state and see themselves as Equal to the "Son of Heaven in the land where the sun sets." This period ended when the late T'ang did the
Great Anti-Buddhist Persecution in 845CE, alienating Japanese elites which went heavily into Buddhism already.
2. Semi-Isolation, with Heian Japan under the Fujiwara regents focusing East, expanding against the Ainu-held Eastern Japan, with the warrior elites adopting Emishi horse archery, and with internal struggles. Ended in the Muromachi period with the first Minamoto Bakufu. But China was divided into two with Northern Liao/Jin dynasties and Southern Song at this time.
3. Hostility, with the Yuan dynasty invasions of Japan, then as Japan went into anarchy at the late 14th century, increasing amount of Woku pirates raiding Chinese coast. Climaxed with the Nobunaga and Hideyoshi's invasions of Korea, actually targeting China. (Well, Nobunaga wanted to conquer China AND India.)
4. Sakoku, total Isolation.
In the second and third phases, there were lots of Chinese monks (or Japanese monks coming to China) spreading their sects in Japan, such as Pure Land Buddhism in the 10th century:
At a later date, the Pure Land teachings spread to Japan and slowly grew in prominence.
Genshin (942–1017) caused
Fujiwara no Michinaga (966-1028) to accept the Pure Land teachings.
Hōnen (1133–1212) established Pure Land Buddhism as an independent sect in Japan known as
Jōdo-shū. Today Pure Land is an important form of Buddhism in Japan,
China,
Korea, and
Vietnam. Pure Land schools make up almost 40 percent of Japanese Buddhism practitioners with the most temples, second to
Chan schools. These schools were influenced by the thought that humans could no longer understand the dharma by themselves.
and Zen Buddhism in the 13th century:
Zen was not introduced as a separate school until the 12th century, when
Myōan Eisai traveled to China and returned to establish a Linji lineage, which eventually perished. Decades later,
Nanpo Shōmyō (南浦紹明) (1235–1308) also studied Linji teachings in China before founding the Japanese
Otokan lineage, the most influential and only surviving lineage of
Rinzai in Japan. In 1215,
Dōgen, a younger contemporary of Eisai's, journeyed to China himself, where he became a disciple of the Caodong master
Tiantong Rujing. After his return,
Dōgen established the
Sōtō school, the Japanese branch of Caodong.
In this timeline, Pure Land monks are going to be heading full-front into an increasingly Islamised Chinese society, and God knows what kind of missionaries and cultic preachers are going to come out of the Middle Kingdom.
Then again, there are loads of examples of dissident sects Islamising a far-off country only for the descendants to return to orthodox Sunni Islam (or Shi'a), the latest example of which is the
African-American Nation of Islam, so...