I would love: Auto-hang up on any call from someone who's number I don't have saved. Just give me a notification immediately thereafter. If it's someone I should know, I'll either have their number saved, or be expecting it. I don't want to be bothered otherwise.
I did this about 18 months ago and love the results. I manipulate and use the telephone network in many useful and interesting ways, all programatically, through twilio.