I suspect that the final product will be amazing beyond my imagination. Not sure if I want 2 or 3 ...or 4. One of my Cayennes has a PCM1 - CD Nav
Sounds like you have the good ideas already, but here's what I think...
No legacy disc stuff u less you can rig a fold - out turntable for vinyl too.
Interface with vehicle for OBD/CAN/ ...similar to Autel MD802 or a good Win10 app if one exists.
Video in input.
The sensors and display for a track/race session as TRK mentioned.
Voice control - microphone input. Would that be Cortana?
I love the pic you have of the console integration. Smooth. Clean.
Anyway, that's my contribution to muck up your thread. I'm starting a swear jar in my garage today to get the funds together. Please target the cost to be a bit less than a base 986 with >100 K miles.