That's a really great question. There is only one company I can think of that has really ever utilized the Reason Remote features of the software and that would be Nektar the designers of the Panorama P4. Their approach is probably how I would approach the MIDI controller. There are three main functional areas (or modes) in Reason. There's the Mixer which has of course: Faders, Trim, Compression, Effects, EQ, Muting, Panning etc etc.
Then theres the Transport Mode which is the sequencer and has the Play, Stop, FFwd, Rwnd, Record, Click on/off, Tempo, Quantizing (Q Rec), and time information. Last there is Instrument mode which has control of instruements and their plethora of parameters.
I already have a basic MIDI control of the mixer though doing more than fading and turning on or off the eq (not even setting the settings of the eq) is not available.
I'd love to see the approach mirror what Touchable has done for Ableton, but that I think would be a HUGE undertaking. Why someone hasn't come out with a Reason controller for iPad yet to the tune of Touchable is beyond me.