Whether picking a career, a niche, a blog, or an entrepreneurial pursuit, there is the underlying domain knowledge.
The underlying domain knowledge needs to be a topic you are interested in the long term. It is a topic that you would explore even if it had nothing to do with making money.
If you are searching for a domain, use the perspective of: What domain would you create content consistently for 5 or 10 years? Pick a domain and create content to build an audience. If you consistently produce content, and you are motivated to create more content you know you have found a niche. The creation of an audience becomes your metric.
When you have found what resonates you will already have an audience so then you only need to figure out how to monetize that audience.