It might be the most difficult, not just politically, but intellectually. Because insofar as the Northwest does better than other places, then the perceived superior quality of life draws more folks from everywhere else. And most of those folks move domestically, not internationally. There is no set of polices that can be used to control their movement without violating their rights. Internationally there are huge ethical, political and intellectual challenges over immigration policy.
Northwest population is still relatively low compared to other parts of the world.
An extremely high priority is to help ourselves and other parts of the world slow growth of population. Then, secondarily and quite a way down that list of priorities for sustainability, we want to make darned sure that we're not subsidizing people to move in. We should completely eliminate all corporate recruitment strategies for economic development. Even from the narrowest perspective of economic development they don't work. It matters an incredible amount whether newcomers move into sprawl or whole communities.