Hmm, going off on a tangent. Sorry.
Dont stop, it's interesting ^_^.
Anyway, the POD here could be no 16th century invasion of Korea by Japan. This would in a way prevent most of the korean isolation, and also remove the number one relationship problem between Korea and Japan.
Korea would be opened to trade by Japan as historically, but without the fear of invasion by Japan, would be much more cooperative (which means that Japan has no reason to invade in the first place. Japans main problem with Korea was that it was too weak against Russia. If they got it, Japan had serious problems), and a strategic alliance against Russia wouldn't be impossible. Japan would probably still be able to invest in railroads in Korea, and would then be able to buy raw materials just as cheaply (if not more) than they got them from the occupation.
By the time of the boxer rebellion (unless it is butterflied, unlikely if the western powers intervene in China as historically), japanese and korean troops might both participate. Japan could get Taiwan as historically, and Korea would perhaps grab some land across the Yalu (not much, but enough to be able to build a powerful defensive line).
By this time, Korea is a dagger aimed at Russia, so the Russo-Japanese war might not happen, or Russia would actually commit much more to it, but the advantage of a strong Korea should help Japan here. Depending on the results of the war, the Entente might look different (successful 1905 rebellion?), but it probably wont change much.
WW1 goes mostly historically, since the Pacific was hardly an important theater. During the 20's recession, the result in Japan and Korea would depend on their forms of government. If both were somewhat democratic, a military takeover in one nation could have the other support a democratic comeback. At any rate, Japan would have many investments in Korea (and perhaps Korea likewise in Japan), so militarism in one country would be met with trade problems, and since this trade would be important, militarism wouldn't be able to get the same hold. Hopefully.
If ww2 happens, I have no idea how it would go. It would depend entirely on the economic situation in the states and china (most probably Japan and Koreas biggest trade partners), but if there's no trade embargo against Japan, I cannot see them attacking the US at least.
This might lead to an interesting timeline, but after the boxer uprising, it is very hard to predict anything, thus meaning that one has to 'invent' history
.