I think sometimes we might want to control channels which are not assigned to any bank/layer.
Ability to select a channel from the touch screen and a fader volume control using touch and turn would help to achieve that.
Behringer Wing has it, just sayin’ 
Beyond that, it would be really useful for something like the C1500 where you have only so many faders & layers & softkeys…
Being able to add an input or output processing channel without having to put on a surface would be nice.
i’ve been solving this for a long time in all my showfiles like this:
i create two DCA groups for all inputs, one with the inputs 01-64 and the second with the inputs 65-128.
so these are two faders in a lower layer, with which i have access to all inputs via the spill-function, without having to put them all on the surface. a small functional help.
the same can be done with outputs, here even a single DCA is enough.
and don’t forget to set these DCAs to 0dB! hoho… 