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I'll start -
Ideal 'Modular fader/knob control surface for mixing' must have/be:
-made up of modular units that fit together physically and conceptually and can be expanded (like Mackie control)
-be extendable backwards as well as sideways, so each channel can get extra controlls.
-in the price range of Behringer fader controller units
-backlit/lit labels on each channel (like mackie control and digidesign command 8 ) with text that is automatically updated from the sequencer app
-an LED based level meter on each channel (like the digi command 8 ) but on a pivot or detachable so they can be viewed flat or in the traditional 'meter bridge' position (like analogue consoles)
-a module of endless rotary encoders (like the behringer ones.) but with backlit labels
-a 'creative' module with crossfaders, touchpads, ribbons, and other similar
-have a quality feel and data output to the knobs used
-have quality high res low noise motorised faders
-transport buttons and scrub/jog wheel etc on the 'main unit' not required on all extenders and modules
-have the standard mute/solo/pan/send bypass/insert bypass lit/backlit buttons and the ability to control surround mixes and others features
-have MIDI i/o/etc and usb ports and some method for chaining units together
-compatible with 'standard' control methods like mackie control and generic remote etc.
-compatible with major sequencer applications, SX, Logic, Sonar, energyXT
-have open source hardware/software and a standard design so any manufacturer can create devices that fit and integrate with it perfectly
-have customisable everything
-not have any audio interface elements and be a pure control surface
-have buttons that display their purpose on the button, like that concept keyboard shown recently.
and other stuff.
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Your turn to design/spec your ideal audio/music thing.