I have one of my Duets wired up to a CNC machine at the moment, though it requires a lot of messing about to from Fusion 360 to actually milling something with the Duet. I can help test stuff out though.
My other suggestion would be support for MQTT, to allow the duet to signal other things with it's status, e.g. current temperature, finished printing etc. With this it would be possible to do stuff like turn off plugs, turn on lights or send text messages when a print had finished.