I've finally decided:
I'll go with John Morrows awesome idea of a New England sprawl, but without NYC and Washington.
NYC will get turned into an ungoverned slum (be it by riots, WMDs, catastrophes and/or whatever), while Washington DC will be an secluded and shielded place surrounded by the sprawl, that is a strict no-go area to anyone who's not a politico, corporate or another high-ranking white-collar.

Thanks to everyone who made suggestions, it saved me a lot of workload.
If any local from New England wants to help me out on the finer points, just contact me by PM or posting in this thread. As it isn't going to become a commercial RPG I can't offer any pay, but I'll happily give credit and link to your site in the PDF and on the homepage.

Well since you making NYC a huge ungoverned slum be sure to watch 'Escape From New York' for some inspiration on it.  The New England sprawl does sound like a cool idea to use though.

Duluth, Minnesota.

Duluth is a seaport at the far Western edge of the Great Lakes, with direct highway access to Minneapolis and St. Paul, it's Wisconsin counterpart (Superior) already merged together thanks to urban sprawl, plenty of crime and Canadian contact, yet through Interstate 35 can (and is) connected to Mexico.  Really, it's a much-underestimated choice of setting.  (Indeed, the whole of Minnesota is that way.)

Cincinnati??
 Plenty of old magic in the nearby hills that could be reawakened.

 If not that I'd vote for St. Louis

Quote from: SkyrockNYC will get turned into an ungoverned slum (be it by riots, WMDs, catastrophes and/or whatever), while Washington DC will be an secluded and shielded place surrounded by the sprawl, that is a strict no-go area to anyone who's not a politico, corporate or another high-ranking white-collar.

I really like the idea of a Washington, DC that's a secluded a shielded place, full of monuments, government buildings, museums, grass, and trees but totally out of touch with everything around it.  New York City as an ungoverned slum sounds interesting, too.  Lots of ways you could do that (e.g., Wall Street crash, corporate exodus, natural disaster, etc.).

Thanks for your further suggestions, but I've already made up my mind

I direct you to Baltimore.

Baltimore would anyway be included as part of the New England sprawl. I guess I can find some use for a city that has a big sea port and is also known as "Body more, Murderland"