And now, lets just add the option for an mATX board (the bigger version essentially is already half way there, 244 mm is the regular size for mATX), and then its the ideal case for medium-sizes SFF builds!
cu, w0lf.
Its not that easy. mATX woudl require a height of 271mm (because of 10mm on top and bottom for the side panel clip zone +6mm inner and outer metal sheets). So its more like an 15.8L case. Furthermore this would only enable 240 AIO support because of the PSU position (facing front of the case). For facing side of the case the length needs to be 388mm. The total volume would increase to 17.5L. So a mATX with classic layout under 17L would sacrifice bigger cooling options or GPU support. If you make the case slimmer all the new cards will also not fit At the end you will just have a big case for supporting mATX without benefits. This is why the A3 is a different size class.