New Cities:
I second SEA-YYZ, SEA-MSY and add SEA-DTW, SEA-CLT, SEA-PHL.
Connect-the-dots:
PDX-EWR
SJC-EWR/BOS
(I realize EWR slots are unlikely, but I would REALLY like to see these, especially longhaul out of SJC, because if AS made it work, the SJC hub could really become a real powerhouse. FWIW, CO does not serve SJC from EWR.)
Additional Frequencies:
SEA-EWR (again, need slots)
SEA-MIA (redeye, ugh)