Honestly, probably something finance/investing-related. It's one of the only industries where profits can be pretty much completely divorced from the value actually created for customers, so there's basically unlimited upside. Everything else ends up somewhat limited by GDPs, population, or anti-trust regulations.

If someone figured out a way to accurately predict market trends in a positive-ROI scenario, they could become a trillionaire without ever having to do all the pesky parts of actually creating a sustainable business.