What would be interesting to see are work visas, entrepreneur/freelance visas, working holiday visas, and so on. Obviously a much more complicated ask but that is what I have not yet seen from a site.
Main reason being this: If you want to travel somewhere you can pretty much go on government websites (either yours or theirs) and see what the visa requirements are. If you're looking to relocate to another country but perhaps don't have a good idea where you want to (or can) relocate to or how, it can be a steep hill to climb. Having something with a bit of a hint as to where to start looking would be nice for people I think.