I like the concept. As usual, having something to attack sparks newer (maybe better) ideas that I wouldn't come up with in a "blank paper" question. With that in mind, a few thoughts. Note that you may have already put these in your mockup, but I don't always get all of the deets from a UI.
- I'd like the audio source to be available not only from a track in my DAW (buffer) and also as something I can load directly into the plugin. Maybe that introduces too much complication, though.
- I like the option of having 4 sources/control sets. It would be very cool if you could split frequency ranges from the incoming audio and send those ranges to the different grain sections (see Eventide SplitEQ or Physion).
- The built-in effects are ok, but I very often bypass that and use my own effects. To this end, the ability to send the four sources via multi-out into the DAW would be a killer feature.
- Similarly, it would be cool to be able to use multi-out in other ways. Think in terms of the Oberheim Xpander's ability to send each voice to a different output.
- With regard to the randomize function, pure random is always fun, but granular can get nasty fast. It would be nice to be able to lock certain aspects of the randomize function, or limit them to a range of values.
- IIRC, Riverrun had a cool function where it would lock grains into a scale or chord. The ability to do this would be awesome, especially if I could feed it MIDI.
- One last thing - with regard to the sequencer, I really like what Slate + Ash did with their new Spectres product, allowing multiple loop/start points and then "sequencing" the program to move to specific points in the audio source. This wouldn't work for the buffer/DAW input, but it would be super cool for a loaded sample.
Thanks for asking our thoughts!