Some niches can be very big, too! It's perfect when it's small enough to not be interesting for BigTech, but big enough to pay a few up to a few hundred employees!
Pick an interest that young white men have. You'll probably find loads of software that caters to it. Pick an interest that an equal number of retired women have. You will almost certainly find a lot less software that caters to it. It is simply because people write software for markets they understand. Most programmers (in my experience) are young white men. A lot less retired women have the skills or motivation to write software.
https://www.reddit.com/r/vintagecomputing/comments/8nzz23/pe...