Here is one way to find ideas for niches,
Check on Flippa, click on established websites, when you see a website that is getting major sales, and you like the niche, look in the description of website, scroll down to how this websites makes money, the owner, will lay out exactly what they are doing to get sales and marketing.
All marketing is shown, from types of social marketing, type of SEO, and tools for automated traffic,
Now the source code, to find out what really going on , check the source code, you can see more, like type of plugin used to do all the cool stuff on the site and much more.
Many affiliate marketers miss this step, you can see what is making money, what niche is making money, and how it was built, so easy. it has always been there, you need to just look.
Now comes your skills, you need to do, reverse engineering, don't copy it , but build the same type of niche, add your style, but stick to the marketing that they did, and design of site, and tools used to build it out.
If you have skills and can do reverse engineering, then you can do this easy,
I have seen many people get ideas, for niche and build out, from Flippa,
Keep in mind, many websites on Flippa are built from pros, so they know what is going on,
So there is your free lesson for the day, enjoy,