Yeah, you're really talking about an entirely different case.
The A4 and M1 use exactly the same 1.5mm aluminum. While it would be nice to just use thicker sheet metal, it would introduce problems to do so. Punching holes in thicker sheet causes more wear on the tools (=higher cost) and increases the probability of panel warping (=higher reject rate=higher cost). You also have to increase the bend radius on the front panel bends when using thicker sheet or else you end up with bulges on the inside of the bend and stretching on the outside, which isn't a good look. Then your minimum flange length also increases with the increased bend radius, so you need to change the design to accommodate for that. Point is, it's not at all a straightforward change like you might think.
This would require an extra 15-20mm of length added to the case, plus some extra height.
For this you're looking at 30mm added to the width, at a minimum, from 160 to 190mm.
These aren't exactly small changes of a "couple of millimeters here and there." Stuff like this is out of scope for an M1 revision.
I do have a design for case - still in the early stages - that addresses some of what you're asking for and that I think could be a compelling option for some people, but it is quite a bit bigger than the M1 (though still SFF, by whatever definition you'd normally apply).
