Let's posit that for whatever reason cotton becomes uneconomical in the very early 1800s. How would the deep South of the United States develop? Could/would they be able to turn to industry similar to the North?
Tobacco, rice, and in a very few places, sugar cane would still be possible.
Also, the Deep South has a very malarial environment. It was not an ideal destination for white immigrants, if it wasn't for the crops that could be grown there, much of the low-lying areas would probably be avoided. The entirety of the US south of the Mason-Dixon line and east of New Mexico was within the range of African malaria, whereas north of the Mason-Dixon line was free of it.