A bit longer battery life.
Level horizon at all times, even while yawing. Maybe auto gimbal roll adjust like that 3rd party app.
User defined boundaries of sensor readings - out of your preset boundaries and the app warns you without you needing to check multiple screens.
More efficient app and/or video processing to lessen the load on mobile devices.
A more robust gimbal that doesn't jettison from the P3 so easily.
An option in the app to turn on some kind of tone, maybe a series of short beeps, as loud as the start up tone. Users could turn on this tone to use as a beacon to assist with finding the P3 if it landed out of sight. Sort of like finding your mobile phone by calling it and listening for the ring tone. Better yet, if the P3 automatically initiates a RTH or low power landing, the beep initiates on it's own to assist with locating. Of course, an option to disable the tone would be needed for those who want or need silence.