I am not a developer, but I am a user of the Moto Z, I love the idea of mods. What I would love to see, and would buy in a heartbeat, is a docking station for a desktop computer, kind of like the one for cars out now that connects to the car's audio and power.
I would want the docking station to connect to the PC not just for charging and file transfers. It should have an audio connection so that phone calls or music are piped threw the desktop's headset or speakers.
It would also be nice if the desktop would be able to access the front facing camera on the phone as a webcam, maybe allow the mount to turn between portrait and landscape positions.
The stand itself should have a weighted base because it isn't clipped to anything, and articulated arm to adjust for best viewing/working angles. Connecting to the PC with USB 3.x maybe with an additional USB-c port for direct outlet charging.
Thank you for taking time to read my ramblings, now get out there and invent something better ;-)