The United States is still ripe with potential areas for future Fallout games to visit -- and for current modders to create. Using a US location would prove a bit easier than some of the places on our wishlist so far, since modders could simply reuse assets like American architecture and the well-established American lore in the game's universe.
Outside of Bethesda's work in Fallout 3 and Fallout 4, the East Coast has hardly been explored -- and the iconic city of New York is a concrete playground that modders can unleash themselves upon. Take plenty of iconic land marks and a massive urban sprawl, then throw in a nuked 1950's vision of the future...and we could easily have a new and incredible place to explore within Fallout 4.
Really though, it wouldn't surprise me at all if The Big Apple turns out to be the next big location whenever we finally get a Fallout 5 announcement.