As far as being as popular as Ball Pythons probably nothing, there are things that are popular or have been popular in the past which meant a strong market Boas, Leopard geckos, Corn snakes all had great years because of the various mutations and then the market changed and the price dropped, does not mean they are no longer popular as pets just means that if people look into popular as far as investment project those are no longer it.
Ball Pythons are still popular but the market has changed as well.
Hognose are popular also not as much and even that market is starting to change.
I don't care much for people who buy something because it is popular, that's not a good reason to buy something, whether it's a pet or a breeder, you need to have a true passion for the animal you keep.