This was an alternate history idea that always interested me for the longest time.
Essentially ( from what I know, anyway, feel free to correct me in anything I say
), the Reconstruction era after the Civil War was a time in which Northern industrialists were coming down to the South to help many of the cities transition from relying on a slave-based agricultural economy to a more urban, manufacturing one.
Aside from New Orleans, which was already pretty urban at the time, this era had the potential to allow many Southern cities to urbanize and built up pre-war landscapes that would be better than what currently exists.
Specifically, I always thought about Florida’s position in all of this. It was the least populous Southern state at the time of the Civil War. It was only after that more of it started to become more developed. Florida also had a pretty strategic location being close to the Caribbean and much of Latin America. Trade in tropical fruits and other products could easily come to its ports and be placed on cargo trains taking it to the rest of the South and to the NE and Midwest ultimately.
Miami today is often said to be the “gateway to Latin America” today, but what if that was true for it and other cities like Tampa, Key West, and other coastal cities far earlier on? What if Northern industrialists like Henry Flagler and others invested in Florida a few decades earlier during the Reconstruction era? Would the cities be far more urban than they are now since they would have grown more traditionally instead of relying on postwar growth? What would happen?