(I'm putting this here because I assume a pre-1900 POD is easier, but the later the better--your call).
Alaska and Hawaii were the last two states admitted into the Union. I'd like to see how it would be possible to make the former's demographics similar to the latter in terms of Whites being a minority.
In the case of Hawaii, the place remained exempt from restrictions on Asian immigration thanks to its status as a territory, for the benefit of the local agricultural elite which relied on cheap Asian labour. Could something similar happen with Alaska, for example on the occasion of its gold rushes?