I'm regularly frustrated by modern phone's complete inabilities to allow any communication when outside of mobile network or Wi-Fi coverage, not even within the two large walled gardens.
It would be so easy for Apple to extend iMessage to work peer-to-peer, at least between people that have already messaged each other before and while both screens are on. That's literally how AirDrop works, and having to send a "Notes" text back and forth is just silly.