A friend of mine shared his strategy with me: find some big uninteresting data set from the government or whatever, figure out some way to make it interesting for regular people, and build a nice interface to it.
For instance, turning old real estate and immigration records into an ancestry site.
There's a lot of stuff in this space that hasn't been done yet, but that's for a very good reason. First of all, it doesn't pass the toothbrush test: it's the type of thing that's super valuable the first time you visit but you don't really need to come back to it over and over.
Second big problem: SEO. Search engines like long form written content, they don't know how to value tools that slice and dice data: they simply don't see any value in it and you'll never get SEO from it.
It is an ugly looking site, the forums are full of nasty comments, but it is still useful at some level I guess. They claim to get millions of page views. Not sure how much money they make, but they have been around for more than a decade
https://www.northdata.de/ Did something like that: Crawl public registries for companies, parse changes to them and build a graph out of it. Very cool!
For instance, turning old real estate and immigration records into an ancestry site.
Monetizing it is still an issue...