@Necere, I was thinking... what if you made a 4-slot version of the NCase M1 that could fit mATX. It's already 3 slot, just add another slot, i.e. another 20mm in height and another 10mm in length, so that the SFX PSU would more comfortably clear the board and you should have mATX compatibility, correct? It would only add ~12% more volume, or 1.5L.
You'd also get a few new options for ITX. You could do a bottom-mounted AIO (might also work in mATX config?). Maybe even bottom-mounted AIO + side-mounted GPU AIO/hybrid rad. You could do a custom loop and cool a bottom rad comfortably with the extra clearance. You could use an M.2-to-PCIe riser or bifurcation riser to add another slot to ITX. Even in the default config, it would be nice to have extra airflow clearance for bottom fans to the GPU. So lots of options, seems worth another 1.5L. And you've already done all the development work and R&D for it, seems like it's just a matter of extending those 2 dimensions a bit.
Does it make sense? I was inspired by those 3-slot mATX boards that fit in the M1.