Very excited about this one.
I've been thinking:
What is the ultimate Control Surface-solution for Reaper/ a DAW ?
The answer depends a little on user-scenario I guess,
but multifunction touch-screen with multifunction knobs seems quite good.
Lot of knobs for most common transport/edit,
and screen-knobs changing for the chosen tasks (think toolbar-icons)
Then, the old paradigm of one fader/ch per DAW-track seems pretty useless.
Personally, one fader (selected track) is enough for most tasks.
Or, assignable faders 8/16 for typical busses.
Then, those faders should work for plugin-automation as well,
but then 2-way communication for led-labels are crucial.
It is of little use having a ton of faders/knobs if you don't know what they are assigned to/ value.
I believe track_ID is sorely missing in Reaper.
But else, most should be doable.
Generic, GUI-less plugins should be ok,
but GR-meter is maybe harder.
Just my random thoughts