During the last days of the Japanese Invasion of Korea in the late sixteenth century, the Japanese had gathered a massive fleet, and faced a slightly smaller allied Korean and Chinese fleet. In the ensuing battle, Admiral Yi, the most influential figure of the Korean fleet, was shot in the chest and died. His last words were telling his nephew not to let the sailors know that he had died. But what if his death was instant? What if, instead of saying this, Yi died immediately after being shot? And his nephew didn't don his armor and pretended to be him? Now, this may or may not mean a Japanese victory if the Korean morale is broken, but we can at least assume that the Japanese do better, and retain at least two thirds of their fleet, whether they win or not. This could mean another battle or two before the end of the war, or a stronger Japan at the end of the war, or at least Japan retaining some of Korea.
This seems to me a great POD for a much earlier Japanese Empire. If the isolation period never happens (the failed invasion of Korea was a contributing cause), then Japan will most likely learn from its mistakes and continue growing in power, at least for a while. Where could this lead?