Does the new city need to have the same "footprint" as the old?

Or could we focus our attention on "building up" a 5-10 square mile area and allow the rest of the destroyed city to return to something that is closer to its natural state?

From what I've heard, the city's population is expected to be about half of what it once was...So the city should be more compact and thus less difficult to protect.

In otherwords, we don't need to protect the entire former city...If entire neighborhoods were wiped out, then maybe the entire neighborhood could be moved to a better location (Ie. higher elevation).