A Sweet Sixteen might be good. 16 faders that can be used for attenuation, offset generation, MIDI control or I2C connection. I do not have one, but it's what I want to fill the space in my rack.
Even if you just see it as 16 attenuators/offset generators, it's a pretty useful tool for centralizing control in a big rack like this. You could then throw in something like a DistingEX, which is normally not very hands-on but has lots of useful effects algorithms and utilities (among other things like sample playback and polyphonic synths), and control the EX with I2C/MIDI to have hands-on access to multiple parameters without menu-diving.
I'm not sure if you wanted that kind of control or maybe a different kind like a touch keyboard, but for the space we're talking about I think this kind of control will be more usable in the rack, whereas that problem is better solved externally so you have space for your fingers to actually do stuff.