I’m going to talk about the idea of blue oceans and red oceans.
Red oceans represent every market that is currently occupied, and the competition within it. For example, it is entirely possible to create a pizza restaurant chain and be successful with it. But it is competing in a red ocean - Dominoes, Pizza hut, Papa Johns Etc…
The opportunities are limited in this market.
Blue oceans on the other hand are a new sector or industry that are unoccupied with competition. They are usually created from advances in technology or inventions. In blue oceans, demand is created rather than fought over.
The biggest blue ocean market that has probably every been created is the internet (split into different sectors). Think of how many companies exist and compete exclusively through it. An example of a new blue ocean that springs to mind is self-driving vehicles. You could say that Tesla and Elon Musk were responsible for creating this. There are currently no true competitors in this ocean, because the technology isn't there yet.
The general idea is that billionaires are created from new concepts or technologies - they come from blue oceans.
So to answer the question, it will likely be a NEW industry that we don’t know about now.
I’d bet on:
Cryptocurrency start-ups
3D Printing
Self-Driving tech
Asteroid mining (might be another hundred years away or more)
Clean Energy (Solar panels are expensive and inefficient)
The idea of red and blue oceans was written about in “Blue Ocean Strategy” by W. Chan Kim & Renee Mauborgne.