Features I would use pretty much every time I was in front of a mix console that wasn't a cold, "one off" mix:
- Delay panning: I've got a single guitar amp with one mic in front of it, and I'm supposed to make it "sound like the record," where it was double tracked, then reamped, and then all of that hard panned L and R. Give me the option to be able to place an input in both the L and R channels, and have the pan knob give me a range of delay (0 - 15ms would be enough).
- Faux Double: Give me ability similar to number one, but retain the level panning position, and give me double of the channel with an adjustable delay time. Throw in a little chorus or other delay modulation to make it feel more like a real double. Use liberally on vocals, acoustic instruments etc.
- Fader level remapping: Once a mix is up and running, with the compression doing its thing, I don't need 20dB fader rides. I need to be able to precisely and quickly make a 1-3dB fader ride. Give me a way to quickly change the range of a fader from "coarse" to "fine." A range of +5 to -10 would probably be enough, as long as the pulling the fader fully down still turned the channel volume to zero. Place fader in coarse mode, get basic mix, then press switch to fine, and the "coarse" level would become the new nominal zero. Fader would then snap to 0dB. Each channel would be hot switchable between modes by a single press.
- Dynamic high pass / low pass: Allow me to set a high pass or low pass on a channel that would change its corner frequency in response to the level of the input. Both raising or lowering the frequency would be allowed, and there would be peak, envelope following, and RMS detector circuits.
- Multiband expander: A pretty common studio tool, it would be helpful to have as a counterpart to a multiband compressor. As an example you could aggressively downward expand high frequency cymbal bleed on drums will only slightly downward expanding the body of the drum sound between hits. Upward expansion of parts of a reverb tail is also an effective tool for shaping ambience. You can also use side chain to key pieces of the mix to pop out as necessary.