I could see DC potentially eventually becoming the capitol again. Philly did get nuked and the main reason that DC lost it's status as the Capitol was the fact that it was really really hard to defend from the Confederates. Moving the capitol back to DC is a final triumphant sign that the Confederacy is dead and the Union is finally restored. After getting fought over in both Great War's it'd need to be largely reconstructed. But the whole damn thing could become one big Monument to the victory of the US an demise of the CSA. That after 80 years of the virus of the CSA existing America has finally been restored (even if the CSA states aren't getting admitted or represented anytime soon). I could also see the Flag being changed to reflect the annexation of the CSA even if those states aren't getting the Vote or representation in at least half a century (unless their's massive ethnic cleansing of those states which are resettled by "loyal Americans".Very good points on urban civil defense in this world NYC.
Another notable feature of TTL’s US urban landscape are the large number of war memorials. As of TTL’s 2009, every major city has its own memorial district. Washington, DC has the feel of a mausoleum city in many ways.
Frankly I think that at a minimum the US will ethnically cleanse Confederate white civilians from pretty much all of the border states at a minimum. It'll be awful but the "success" of deporting the Mormons to Hawaii and the struggle to police the entire former CSA will wear on the already very very tired US. I imagine the US will do a new "Homestead Act" in the states that will be ethnically cleansed (Virginia, Tennesee, North Carolina, Oklahoma, Arkansas, and the like). Former US Army veterans, black survivors of the Population Reduction, immigrants from friendly nations, the dispossessed from the destruction of the Confederate invasion of the US, and the like would be resettled. A little like a combination of the Indian Removal Act, the Nakbha, the expulsion of Misrahi/Sephardim from various Arab countries after 1948, the expulsion of ethnic Germans from Eastern Europe after WW2, and Stalins various "Internal Deportations" of ethnic groups he considered unreliable. The expelled white populace would be deported to a handful of former Confederate states that are stripped of all industry and heavily garrisoned by the US army. They won't ever be allowed to be Independent but won't be formally annexed anytime soon. Basically a couple of massive reservations. You might also see the formation of a new semi independent homeland for black survivors of the "Population Reductions" from one of the former CSA states
It'll be awful and horrific in a lot of ways but I think it'll be more realistic considering the general perspective of this version of the US after Four wars with the CSA in 80 years and millions of dead (and all the destruction of the CSA invasion of the US in the SGW). Actually trying to police the entire former Confederacy would be a nightmare which would take a truly massive army even with the brutal tactics this US uses (Like responding to an insurgent killing a US soldier by taking several dozen or hundred innocent civilians hostage and then executing them). I think they'll eventually decide that if they can't find the insurgents in the sea of the Confederate populace across over a dozen states they'll reduce the number of states needing policing to a handful.
In terms of the pre war US/CSA border I imagine there's almost continous fortifications of various types that got built gradually over 80 years. Many would be obsolete or destroyed in the war but in the long run after the war you'll still be looking at a almost continous stretch of concrete, barbed wire, landmines, brick, and earthworks. Many sections might end up being preserved as National Parks.