It's a bit broad thing...What should it exactly do? What isn't already possible (in theory) using Csound's VST plugin version? (Theoretically, because at least the last time I tried, Csound VST was quite unstable in Reaper.) Isn't Reaper's built-in JS language good enough for some DSP experiments? (Not dismissing your proposal here, just pondering...) I've myself included one action in my commands Reaper extension plugin that calls csound.exe and performs some simplistic offline timestretch/pitch shift processing on a selected media item in Reaper. (I did it because the Csound based processing allows arbitrarily long stretching as opposed to some of the stretch modes in Reaper.) I suppose I should try making that use the Csound API directly, just to see how complicated it might get.