Check-ins could go far beyond just BTCMap and be a full Foursquare (before Swarm) alternative. I was thinking about that use case recently, because I miss using a check-in app for essentially collecting place bookmarks, like I was doing in the mid 2000s.
BTCMap could then also integrate that functionality (or do it first) and be an entry point for new users to find the check-in app(s).
I think one important aspect that was normal for centralized Web 2.0 apps, but is difficult to design on Nostr, is various permission/visibility levels for certain details of data entries. For example I want my approved mutuals to see where I just checked in (and even get a notification if it was close to them), but not anyone else. A single service with its own user database can easily manage these kinds of permissions, of course.
