I think low and mid-rise apartments/condos/townhomes will be sufficient enough for most American cities in 100 years. Surface parking lots and commercial-only areas will be replaced by denser mixed-use developments and residential-only areas will probably densify too with some single family homes being converted into townhomes.
Here is a side-by-side image of Phoenix (metro pop. 4.2 million) and Mexico City (metro pop. 21.2 million) at roughly the same scale:
Mexico City has a few high rises but not many considering how big it is; most buildings are probably in the 2-6 story range. I think it has an ideal density (15k/sq mile) for a city its size, and I think cities like Phoenix, Houston, Atlanta and Dallas could (and should) reach 20 million without expanding any further outward on their own footprints.