I'd note this is already possible on Android, using 'Smart Lock' - https://support.google.com/nexus/answer/6093922
Quoting GP: I think a good balance between security and usability would be to allow fingerprint or iris scan when the phone has been constantly in my proximity but require a pin (password) if the phone is taken away. The proximity could be determined for example by pairing the phone with smart watch.
When combined with a fingerprint sensor, smart lock keeps the device completely unlocked while "triggered" (by being on-body, or close to a trusted BT device, etc), and the fingerprint unlocks it while not triggered. It doesn't ever escalate to requiring the pin/password/pattern. Please correct me if I'm wrong, because I'd like to be.
Definitely not a perfect system. I wish that I could set timeouts and map the power button to do an admin lock. Also, having to use a 3rd party app for this is quite likely its own threat vector.
[1] https://play.google.com/store/apps/details?id=com.katecca.sc...
I'd love to have features where the fingerprint is only good enough under certain circumstances, such when the phone hasn't been idle for too long, or when combined with an RFID tag.