I think It would be a good idea if a Duet3 CNC oriented board came out. RRF is great, and very easy to use, but as mentioned before all the boards are very 3D printer oriented.
I would like to have a Duet3 without drivers, thermistor inputs and heater outputs and instead have dir/step outputs and ethercat for the use of industrial servos. Another missing feature is an encoder input for a spindle, needed to do hard threading, and an addon board with isolated inputs and outputs needed for a CNC. Also an MPG input.
Software side, there are no fixed cycles (for now at least), which is not very necessary for a mill (or a router), except for drilling cycles, but it is really used on lathes, also tool radius compensation is needed for a lathe. Otherwise, everything needs to be done on CAM.