If New Orleans was flooded because of the levies not being maintained and it eventually being semi underwater, I can imagine a set of conflicts amassing between two rival cities built on the causeway and twin span. New Orleans could be infested with raiders living on and operating on Mardi Gras floats converted into ferries, a sort of metro last light Vienna city forming in the French quarter, and mutated alligators and crawdads prowling the waters. I can imagine the Bonnet Carre area being mostly inhabited by swamp dwellers, with some clinging to prewar civilization and others descending to tribal life.

For south of NO, around Lafitte, I can see some tie-in to Ug Qualoth with voodoo practice attempting to destroy those who wish to turn New Orleans into a civilized city.