It's not original - in fact, it's published and part of a series, but...
Imagine this: It's 1274. Kubla Khan's armies have swept the Japanese from the field, but they have retreated in such good order the Mongols don't know if the sons of Yamato are just going for more reinforcements, or are really fleeing.
POD: The typhoon hits early.
With no choice but to weather the storm on land, the Mongol army marches inland, following the Japanese. Days later, reinforcements slowed by the sudden storm, despite years of preparation, the massive landing of the Yuan are no match for Kamakura.
Within five years, all of Kyushu is a province of the Golden Horde, Shikoku follows soon after.
1281.
The typhoons blow late, instead of early, and the Yuan, with so many places to land on Honshu...choose them all.
With their lands in ruins, the children of the rising sun flee, boarding ships and setting sail, east, into the Great Sea.
Where they receive a miracle - land!
Ladies and Gentlemen, I give you the Nisei-Mexica!