I'm just going to start making observations as I go through this thread.
The first thing I noticed early on was that your buildings all look very similar, too similar. But, maybe you change that later on, so I'll wait until I reach the end to comment more on that.
As to Washington D.C.? My gut says don't make it right now. Maybe later, but for the time being I'd worry about getting the main basics of you mod figured out. Later on I would consider it.
And since you specifically ask about what we think your mod should do--regarding being the USA or even Earth--I would say, make multiple versions of the same mod. Again, first start with the basics, the framework of the mod, that could be applied to any post-apocalyptic planet. When all of that is done, release it as a standalone mod. Call it Ghost Coast Apocalypse or something.
Later, add in Washington DC, the Statue of Liberty, whatever other things you want that are specific to Earth. Release that stuff in a second mod called GC Apocalypse: Cool Scenario Name Here.
Gives players more choice when it comes to deciding what they want to play.
Buildable world wonders:
Mad Max Thunderdome
Half Life 2 Citadel and Black Mesa East
Natural wonders:
Walking Dead--Center for Disease Control & Prison
Planet of the Apes--Statue of Liberty Hand
The Postman--Bridge City
Zombieland
Half Life--Black Mesa
Left 4 Dead--Mercy Hospital
Left 4 Dead 2--Whispering Oaks
Resident Evil--Looking Glass House
28 Days Later--Balfron Tower
The Mist--Arrowhead Project
The Tripods--White Mountains
Lots of cities as well you could include in the mix--City 17 from Half Life 2, Bridge City and Pineview from The Postman, Raccoon City.
Regarding trees, I'd say reduce their numbers. But you might also consider another mod with few or none.
Eras
1: Survival
2: Fighting Back
3. Rebuild
4. Last War
I don't know where you've looking besides Fallout for ideas, but here are some I'm familiar with:
Games:
Rebuild 1 & 2
Half Life 2
Neo Scavenger (mostly forests, you may look at the buildings in that game for inspiration)
Caravaneer 1 & 2 (almost entirely desert settings)
Movies:
The Postman
World War Z
I Am Legend
28 Days Later
Shaun of the Dead
Dawn of the Dead
I'd look at some pictures of these games and movies.
28 Days Later Henry West great general, probably lots of others from other media
President Madagascar from game Pandemic.
I am legend Great Doctor Robert Neville--you could have a zombie virus spreading through cities or something, and doctors would help to treat the virus, with great doctors getting closer to cures to varying extents.
Feral ghouls--can't you just have the game randomize which type barbarians get?
Ok, I finished going through the thread. The big thing that stuck out to me was that the tone of gray for your buildings is too boring--I would have more variety, slightly different tones of gray, brown, etc, instead of the same color for all your buildings.
Ok, so I think that's good for now, should serve to give you some fuel for your mod for a while