OK, so are there any alt timelines were Japan colonizes the portion of North America west of the Rocky Mountains? Preferably not the only results I can find about Japanese western USA - The Man in the High Castle.
I doubt they could expand past this area before the USA come knocking. And obviously the shipbuilding and other technology would have to be in place when the Japanese decide to sail east. Would they beat Columbus?
Remember, Japan didn't become isolationist until the 1600s. Here it wouldn't even come up in conversation.
This would cause major differences in the factions during the Sengoku Period (assuming it's still called that), as different daimyo hold on to different colonies in America. The areas that fallen daimyo rule over could claim their independence until the group as a whole becomes a new nation, separate from Japan. The resources of western North America could allow them to be self-sufficient, especially if this nation still exists when gold is found in the 1800s.
With a self-sufficient nation over the fence, how would the US react? Would this nation even end up becoming states, so no USA? Or just midget USA? Would the racism of the time whip up Americans so much that they elect a war-hungry president who believes fully in manifest destiny?
If WWII went the same way as in our timeline, the large amount of Japanese descendants in the US could lead to even more large-scale anti-Japanese riots and prejudice.