I´ve been reading a bit off japanese history lately. There is a lot of fun ATL material in the time when the portugese arrive to Japan. First of all, the outcome of the power struggle between different Daimyos may vary, and the religious culture of Japan is seemingly open for change. Of course this has been discussed before.
However, I´m not interested in a Nippon-wank where Hideyoshi´s christian army marches over Korea or something like that. More subtle kind off a TL.
Like, Obunaga conquering Japan without being assasinated and even convert to christianity. Not a TL where every Japanese becomes Christian, just a TL where Japan is open to trade and where christianity gets a permanent foothold. After Obunaga´s death, the next Shogun might be hostile to christianity, neutral or at least in any case not capable of eradicating it, and still interested in continuing trade with Europeans.
A Japanese version of Christianity arises. (Perhaps in 1750-70 someone has a mormonlike revelation and claims that Jesus came to Japan after the crucifiction). The japanese sects become influenced by buddhism and shintoism and their missionaries cross over to Korea and even China. At the same time the catholic church has been operating for over a hundred years in Japan, with maybe half a million followers, a few bishops and is able to support missionary activity in Korea.
Maybe a Shogun in the early 1800s has an interest in navigation (maybe the kind of interest that the Ming emperor that sent Zheng He had, rather than economic interest), he hires westerners to build ships for him or maybe just buys them from Europe and sends a mission out into the pacific. (In this TL Japanese settlers overtake Hawaii and perhaps arrive to the coast of California long before the time of commander Perry).
Of course there´d be several butterflies and the world becomes different, both Korea and Japan industrializing at similar times... Japan becoming a major sea power, winning Spain in a war in 1870, claiming the phillipines, Japan and Korea allying to fight off Russian aggression, then splitting northern China...
How does it sound, ideas? Criticism? Suggestions? What would be the most plausible sounding way to pull this off?