How about a POD of between 1543-1614 between the time when the Tokugawa consolidated their power in Japan to reign in all the unrully feudal lords, and when Portuguese traders and missionries began arriving to Japan and the Christian missionaries started winning a significant number of converts among the warlords. These Feudal Lords known as "Outside Lords" were extremely antagonistic to Ieyasu, and the Outside Lords in TL also had a large number of Christian converts among them.
In TL the Tokugawa began persecuting the Christians in earnest, because it was strongly associated with the Outside Lords who were a real threat to the Tokugawa. In particular the Samurai class presented a particular threat to the Tokugawa especially the masterless Samurai. I think the Samurai is key to curtailing or eliminating the Tokugawa, by having a large number of them convert to Christianity and swearing allegiance to the new class of Christian lords.
These lords controlled a number of key ports who opened them up to the Portuguese in exchange for firearms, therefore an alliance between the newly Christian nobility, the Portuguese, and their subject population aswell as the Ronin could form a fifth column against both Togugawa.
In TL there was in fact quite a number of nobles who became attracted to Christianity.
Let's say the Jesuits were extremely successful in converting the aristrocracy and once the aristrocracy converts, the rest of the Japanese commoners would follow.
The newly Christianized Japanese lords starts awarding the Jesuits with consierable sway at the port of Nagazaki and others.
By 1600, close to a half a million Japanese from all classes have converted to Christianity.
In OTL 1616, Hidetada engaged in a merciless persecution of the newly Christian converts. 30,000 Christians made a final defense along with the Ronin. The Christian stronghold of Hara Castle where some Christians took refuge was bombarded by the Dutch by sea, while the shogun stormed the castle and massacred the defenders. By 1639 the Shogun had closed the country to outsiders.
Perhaps a Portuguese intervention to lifet the siege by sinking the Dutch ship could help save the Christian defenders, and the combined army of the Outside Lords can turn the tide against the Tokugawa and keeping Japan open to foreign trade.
This can have a long term effect on the development of firearms and other technological innovations in Japan.
The Chinese could then probably reverse its own isolationism by opening up to the West.