App idea: show me everyone who's checked into the same places I have

This app idea came to me when I spent 3 days in New York City and realized that I would have probably been in the same subway car as a few thousand people that I follow on social media, but I wouldn't have any idea who. That's a good thing, because of safety/security/creepy stuff. But, if people could opt-in to a universal database of "I'm over here" that would be pretty cool.<br><br>The app would let you use GPS to mark your location on a map, then show all the public check-ins from anyone else who has used the app. You could then filter by your friends/followers/etc. Or, if you wanted to be notified when someone you know was in the same place as you at the same time, you could set a notification for that.<br><br>I can imagine this being useful in a few situations, like conferences (if you wanted to find other people with similar interests and talk to them face to face) or sporting events (if you and your friend are in different sections, you could coordinate a spot to meet up for a beer). <br><br>I think someone tried to do something similar with foursquare a long time ago, but this would be more focused on discovery of other people, rather than the location itself.