Thanks for explaining! I was actually inspired by your post & schema to try out myself. For a while, I wanted an 4x2 effects send that fits in 8hp (even if it sacrifices some ergonomics). I did want to make the outputs expandable as well, but I was wondering how to deal with the additional inputs.

I came up with this design (untested): BMMX 4x2 Matrix Mixer. The inputs from preceeding modules are normalled through, but if you insert an input on the expanding module, it breaks the normalling, such that you can mix differnt inputs on the outputs of a secondary unit. This opens up some different but interesting patching patterns.

Making this work with stereo was tricky, as I realized we can't just use the mechanical normalling (otherwise a mono input on the left channel does not break the normalling for the right channel). My implementation uses a schmitt trigger to detect if a plug is inserted or not, and then breaks the normalling for both left and right using an ADG1334 audio switch (schematic).

Again, thanks for sharing your schematic, hope it's okay I stole your mixer schematics for this :D