My vote is:True bed levelling using 2, 3 or possibly 4 independently-driven Z motors and a Z probe.
This one exists now. https://duet3d.dozuki.com/Wiki/Bed_levelling_using_multiple_independent_Z_motorsAutomatic advance of changes in the colour mix when using a mixing hot end. See deckingman's blog. Support for PanelOne 20x4 text displays, or possibly 12864 mono graphics displays.
The Duet Maestro supports 12864 displays. The other Duets lack the hardware needed. But there are other alternatives to the PanelDue now as well. Cheap tablets as a web interface or even a customized simplified interface. https://forum.duet3d.com/topic/9257/dueui-1-1-0-is-releasedAbility to update PanelDue firmware via the web interface.
Working since RRF3.2 😂
Ability to control an electronics cooling fan thermostatically based on CPU temperature and driver overheat warning.
This is now possible as well. https://duet3d.dozuki.com/Wiki/Mounting_and_cooling_the_board#Section_Cooling
Thanks for all you do for the community!