As a theoretical, how about medicine, pharmacology, every flavour of engineering, fine art, music, architecture.
Coding, graphic design seem to have a few things going for them that make them a very natural fit for self learning online. Part of it is not needing much outside of a laptop and an internet connection, but math needs even less and you find far fewer autodidact mathematicians.
I suspect that not needing a curriculum is actually a part of it. I don't dispute that having a curriculum available to you can be useful and that following a curriculum will get you from start to finish will probably give you a relatively efficient, high success rate path. But, I think that the more self contained and small an effective quanta of learning is, the more suited it is to self learning.