Maybe something simple... like the entry door into the garage having a WiFi and/or RFID lock. So the door into the home, always locks behind the person leaving... but is also always open to the authorized person entering.
Simple, I'm sure you're being facetious. We'll have to embed an RFID chip between your shoulder blades, install an RFID reader on each side of the door to determine direction, then....
Simple??
You want simple? I have one of these on every interior and exterior door, even the shed outside.
http://bit.ly/2FRdOer and a few of these
http://bit.ly/2Q55Nap scattered around the house. Check the size difference compared to a z-wave motion sensor. And, the z-wave is even smaller than the X10 sensor.
https://photos.app.goo.gl/VNR3uZ37p4WZbNqG6You should look at getting a zigbee board to attach to your pi
https://amzn.to/2ACYj3K. I have door sensors, motion sensors, water leak sensors, all for a fraction of some X10 or z-wave stuff. Obviously nothing beats what you already have, but if you're thinking of changing or adding...