There are a decent number of stories with China as the world's greatest nation, few with Korea ascendant, many more with Japan. Two series based on the voyages of the Treasure Fleet turning out differently are Chris Roberson's Celestial Empire tales and Aliette De Bodard's Xuya Stories, says
uchronia.net. Haven't read them, though I've read the "Japanese-flavored Aztec Empire" Sixth Sun series by Thomas Harlan (and googling, I see the third volume came out a year ago and I didn't notice it).