While I have no interest in anything that would automate this without some kind of clear opt-in (user privacy), nothing stops anyone from posting such information about themselves in a introduction tag or in their bio, which you can currently view by hovering over a username in a post.
Of course, something that separates this into some kind of searchable manner (eg find all in a given city) would be very cool!
As JTL said, this would fall info a Flarum extension, so requesting it here would be a good first step