Here are two shots of a model of BMO Field (home of Toronto FC) placed in the north end of the Starfire Sports complex. Assuming we can find some public funding for one of the cheaper recent MLS stadiums and put it among the largest soccer complexes in Puget Sound, building an MLS stadium sounds feasible.
Here is an article from the Seattle Business Journal (thanks Goal Seattle) about a plan to build a stadium on the location in the shots below.
From the north:
From the southeast: