What if the outcome of the Russo-Japanese War had involved a division of Korea between an independent, but Russian-dominated, North and a Japanese South?
Before the Russo-Japanese War the Japanese proposed dividing northeast Asia between a Russian-dominated Manchuria and a Japanese Korea. Russia instead wanted Korea North of the 39th parallel to be a neutral buffer.
Maybe North Korea could be a neutral buffer rather than under Russian influence? It more or less serves the same function.
I'm not quite sure why Southern Korea is so indefensible. The whole Korean Peninsula is fairly mountainous and the main point of a Japanese Southern Korea is for Japan to have buffer space to protect the islands from. The Taebaek Mountains, Central Korean lakes, Rivers, etc make for an okay northern demarcation.
At the same time, this could open up an interesting front in World War Two; would there be fighting between Japan and (a) USSR, come the Second World War, if both these things are not butterflied away?