I know that this is probably, at best, a very long term feature request, if even doable, but here goes …
Allow a track to be a MIDI sequence recorded by a specified MIDI Channel and sent out on another defined MIDI channel. Mixer volume control of the track would actually control the MIDI Velocity (for dynamics) or, I guess actual Volume if that’s what the user wants. Maybe even just 1 MIDI track available in each Song Part (so 6x6 would actually be 6x(6x1)).
There are some gigging backing track devices that stream both audio and MIDI, but I’ve never seen that in a hardware looper before. That would be a dream come true.
I guess there was a feature request similar to this already, should have searched first: Midi record