I'd also like to be able to visualize the contents of the tracks in a particular way.
I think Cinder will probably be sufficient for building the UI and doing the audio recording and mixing. I'll probably write my own MIDI layer, and I'll probably bolt on Lua for implementing the sequencer's high level logic.
If this ends up being a good way for me to make music I might try to incorporate it into Tangerine, since the general scheme here could be tweaked to make building dynamic soundtracks really easy.