A good example would be Paul Graham and Lisp. A commenter on here was saying Graham's startup that was sold to Yahoo could run circles around his competitors, even those with 5x the budget, because using Lisp gave them a huge competitive advantage when creating web development templates.
Or, tl;dr: use a Lisp to create DSL's (domain specific languages) in some commercial niche.
Another example would be Erlang and building a top-tier, low-latency communications app.
The definitive example is WhatsApp, but as one person, perhaps you could create a mind-blowing prototype, or get a lot of traction just by being faster than anything else in some related niche.
What are some other examples, especially ones that are relevant today?
Incidentally I'm a consultant using JavaScript, React, and AWS, so I'm not looking for a job or the obvious answers (Python for data science, Typescript for static typing, etc). I'm most interested in the hardest but perhaps unexpected or generally unexplored languages/stacks/etc. you can learn, that would launch you ahead of the pack by doing so, and maybe make products possible that aren't commonly found otherwise.