I heard a rumor about SEA gates after the IAF is completed in the Delta gates thread. Supposedly, in 2019, WN, UA, NK, B6, and F9 will move to S (maybe Sun Country as well, as they didn't mention them?) and Delta moves out of S. Here's basically how it would look:
D: AA, AS
S: International airlines+WN, UA, NK, F9, B6, SY
I think this could happen, personally. I don't know how it'll affect gate congestion, but at least it organizes the airlines more, without upsetting AS, and gives DL room to expand.
I would agree with pretty much anything here except I don't think WN will move out of B. The Port is going to want to try and attract as many international carriers as possible. Moving WN from B to S would put a major dent in that plan.
If UA moves to the S-gates, then I'm sure AC will do the same.
Lastly, when the N-concourse expansion is complete, there could be a possibility that AS could leave a couple of gates in D concourse for B6, NK and others...but just a possibility.