Ideas to find a niche for self-publishing

It comes across like an SEO problem, I will just add my two cents. Within my sphere of making websites for the current thing, the issue is everyone jumps into the largest 'market', of course for economies of scale. But a lot of smaller markets get neglected. To make it concrete, when I pursued the "web hosting" wave 20 years ago before it became a commodity, it was tough to stand out on the generic terms, BUT at the time people always gravitate to local solutions, so the tail end searches had a lot of local searches like "web hosting canada" or "web hosting montreal" or web hosting toronto". So placed my efforts on ranking on these tail end keywords, including have a domain with those specific keywords and basic articles and reviews relinking to those keywords. Within 6 months it generated a very decent business in web hosting space in those early days.

Other idea I heard from a friend is the neglected platforms. Like kindle. Everyone makes apps for say andriod or iphone sphere, same arguments, economies of scale, and they neglect the much smaller but still very profitable sphere of the smaller networks

Hope that generates some ideas on your end, good luck