We did work last year on MIDI alone - https://openai.com/blog/musenet/ and some early work now on conditioning the raw audio based on MIDI (early results at the bottom of the Jukebox blog). Agreed though there should be interesting results from modeling different blends of MIDI, stem, and raw audio data. Raw audio alone gives us the most flexibility in terms of the kinds of sounds we can create, but it's also the most challenging to get good long term structure. Still lots more work to be done!