The Twitter microblogging service has the ability for clients to "stamp" updates with a geolocation. If they would extend that just a little bit, some really actually useful stuff would become possible.
Some of this would need features in the client, some would need features in the server, some could be done with work in both sides.
I would like my handheld client to give me a special alert when someone I follow does an update that is geographically near me. As it is now, my client in my phone only alerts me when someone DMs me or mentions me, as it would be entirely too noisy if everyone I follow does anything.
It would be useful to be able to subscribe to the public timeline of some space. For example, then a conference could put up on public monitors all the local tweets, not just the ones that match a hashtag.
Another neat thing would be if I could set some metadata in an update that means "this is probably only interesting to friends near me". The service then could either filter it, and/or pass it out to my followers, but their client could choose to suppress it if it's not near enough. How near is "near enough" could be set as a hint in the metadata, and/or a default my account settings, and/or by settings in my follower's clients.
Labeling a location would be neat. For example, it would be neat if I could right now tell Twitter "loc Remedy", and instead of a generating a public update, the service says "hmm, from geoloc data and past history and past updates, I am pretty sure that he means "Remedy Tea; Capitol Hill; Seattle; WA; US". So now my location has a name. Which can be fed back into the public latlon-to-name database as well.
Another useful inline command could be "checkin". If I just say "checkin", then a geolocation tagged update would go out saying something like "is at $PLACENAME", where the placename is pulled from a latlon-to-name database, and/or from my recent or past "loc" history. The "loc" and "checkin" commands could even be combined, so if I send "checkin Remedy", then my local placename gets set and refined, and the announcements goes out as well.
If Twitter and/or the other microblogging services get this right, they could very well "eat" the specialized location/social services such as foursquare and brightkite.