That being said, here are some features I'd like in a "perfect" controller:
Dedicated button bank for cue selection
Separate button bank for chases (need enough for projector chases and BEAM-RAIL turret chases - maybe separate banks?)
Cue page swap button(S)
Joystick for position in X and Y with dedicated "reset to home" button
Rotation knobs for X, Y, and Z (both discrete and continuous rate) with "reset" buttons below (or above)
Dedicated button bank(s) for effect selection (thinking about the "Beyond" interface here)
Sliders for scan speed, track brightness, overall brightness, effect speed, color cycle, etc
Master blackout switch
Flash buttons for projector channels (like chases, but momentary on)
Tap button for beat-sync
Figure on at least another half-dozen user-configurable buttons that could be used for various assignable commands, and the same thing for the sliders and rotary knobs...
Not sure if it's worth it to have LCD's for text-assignable labels - I have a feeling that would add too much to the cost. But having custom skins available would be great.
Basically my thought is to put all the controls you might normally use from the keyboard, plus all the controls you currently use on any controller, plus the extra goodies all in one box. That's my suggestions anyway, but I have a feeling all those features will make it too expensive...
Adam