There is some really great African food up north just outside the city. :-D Likewise along the northern parts of Aurora you can get some really great Korean food. (Also not strictly in the city limits).
Parks, lots of parks.
The only decent real "spas" I've found are all up north (Again, Shoreline, just outside of the city)
Federal Way has good Korean food also (well, along with lots of Koreans). Frankly, you'll find Asian communities south, east and north, with the high end in the east, the middle end in the north, and lots of value in the south.
Schools are better in the north, which is why we chose Ballard rather than Beacon Hill. The reason I don't think about North Seattle so much is because I live here, I guess (and getting places isn't so hard if I'm not crossing Seattle).
It sounds like you are already set up for I-5 getting obliterated.
If you only go to the south end for Ikea, Southcenter, and Seatac, how often do you really need to do those things? It's not like it becomes impossible to drive there, it just takes a little longer. Frankly, traffic in Seattle just isn't that bad compared to other major cities. I just checked the traffic from Ballard to Ikea, and it's 45 minutes on 99... I-5 isn't even the ideal route right now.
Of course I'm playing devil's advocate a little bit here, but you also have to weigh this against just how much additional real estate would come available if I-5 was gone. I don't think people's need for driving convenience actually stacks up that strongly against all the other positive considerations.
Whenever I-5 is blocked off, 99 will be a mess, so its not like I can just say "oh, not my problem because I can avoid using it." You have to think about the traffic between Vancouver BC and Portland Oregon, it has to go somewhere, and 405 is even worse than I5 most of the time.
Parks, lots of parks.
The only decent real "spas" I've found are all up north (Again, Shoreline, just outside of the city)
Ballard and Fremont are both big draws.
UW, kind of a biggie.