I think it will be SEA.
No current Oneworld partner flies out of SEA to Asia and JL would be the first.
SEA to Asia traffic is growing a lot and would feed into JL routes.
MSFT, AMZN have HQ there and population is booming.
Lots of companies and disposable income to sustain pax count.
AS is also a good partner and the main hub at SEA would provide connections to other parts of N.America.
It wouldnt be SJC as there is SFO nearby.
Wouldnt be PDX, PHX, LAS as there isnt nearly enough traffic to Asia.
But i'm a bit biased...
