This is the full feature list for a check-in and live-escort safety app, based on a working build. Mark each feature as part of the first version, something for later, or not needed. Every section has space at the end to add features of your own.
The core idea behind the app: the countdown runs on the server, not the phone. If the phone is broken, flat or out of signal, the alert still goes out, because silence is what triggers it. Keep that in mind when choosing what to cut.