Yeah, Ryukyu. It's that place that shocked many Europeans, mainly because they didn't believe that such a militarily weak state could remain independent.
With a post-1400 POD, have Ryukyu be a flourishing successful sovereign state today.
Bonus: Ryukyu conquers Taiwan
Bonus bonus: Ryukyu conquers Japan
Bonus bonus bonus: Ryukyu conquers China
Bonus bonus bonus bonus: Ryukyu is world superpower.