Don't hold your breath on B&C. SEA's long range expansion plans are to extend the D concourse northwards, connecting with the N satellite and beyond.

Before 9/11, the Port had plans for a whole new terminal building (including landside facilities) to be set way north of the north satellite. Now the plan will be to extend D incrementally as growth warrants. See this article.

Growth currently doesn't warrant even thinking when they might start as there are plenty of empty gates in the new A extension.