I've always thought it'd be a great thing to have an app or service that acts as a "lobby" of sorts to pair you with like-skilled folks for workouts, pickup sports, runs, etc. For instance, if I run an 11 minute 30 second mile, I'd love to be paired with someone who also runs that time, so I can enjoy the activity with someone without them getting mad that I can't run a 7 minute mile. It's a feature of online gaming matchmaking that I think could work well in the real world.