As much as I'd love to see more service between Seattle and Europe, I don't think it'll happen. Very unlikely for any additional destinations in Europe from Seattle due to problems with connecting traffic.
On the US side, SEA is not a logical connection point for vast majority of passengers.
On the European side, look at the Skyteam hubs:
AMS and CDG - already served from SEA, lots of connection opportunities. CDG drops to 5x a week during the winter.
FCO - Most logical choice if they had to add one, but doesn't open up a lot of connection opportunities that aren't already well serviced via CDG or AMS. For most of Europe heading to Seattle, connecting in CDG or AMS makes more sense than FCO
MAD - Not going to happen. Same problem as FCO with regards to connections. You'd see DL service to DTW or MSP first (and I don't anticipate them adding it)
PRG - No times a million. DL only has seasonal service from JFK.
OTP - No times a billion.
No DL service currently.
For any other Seattle-Europe routes, DL would be entirely dependent on local traffic. LHR is workable, but even that is tough with the competition from BA who has the advantage of carrying both local + connecting traffic. What other markets would be big enough?
FRA? Not a chance competing against LH on this route. Would be more practical to light piles of cash on fire.
DUB? Sure, Amazon and Microsoft would put some people on the route. But even then, that is maybe 50 business travelers (some in economy, some in business) roundtrip per week on that city pair. Even if you didn't do daily service, only running it 5x a week, you'd still need to find a way to sell another 1500 seats a week to make it viable. Not going to happen.