Having commuted the I-5 from S. Everett to downtown in the past, this is totally obvious. Next lines would need to go between Belleuve and Bothell along the I-405, and of course the 520.
I don't care how much these lines would cost, they should just be done and get started now.
Actually if we want to speed transit to an air hub and serve the region we need to reopen Paine Field to commercial air travel instead of funneling down to Sea-Tac for all travel.
As for the North-End line, totally agree, Why they couldn't have built in the I-5 right of way originally, hell let's run the trolley on the old interurban line