What you are looking at is my China Playthrough of the PRC that I.. had 'fun' with (used console commands, because I barely understand HOI4 that isn't LARPing as leftists and gaining super weapons lol) This is in The Fire Rises Mod right here, that features cool and weird paths and allows for these fun stuff to happen. What you see is just the beginning btw, because this: 
is what I ended off with before I went to bed. Took me like, 6 hours, honestly, and I need some.. break from HOI4 afterwards, because being this obssessive over moving units around does not make for good creative work, but if you want, I can like... add more screenshots of the Americas, or Europe if you're curious on how that went.
The ROK would actually make more sense as a leader in the PTDO war effort, given that the ROKA is a well-oiled machine by comparison. Sure, Japan has a moat defending it called the Korean Strait but in that case, Australia and Indonesia would be contenders. But overall I agree.