This would support taller cards than the M1, though how much taller exactly I haven't determined. It would be nice to add a little more room behind the motherboard for cables, but that's directly at odds with tall GPU/CPU cooler height and/or case width.
Since you mention the MSI Gaming X, I'll use that as an example. Consider two cards from that series:
MSI GTX 1080 Gaming X:
MSI GTX 1080 Ti Gaming X:
Both of these have PCBs that are 140mm tall, but only the Ti has recessed power connectors. Those need about 20mm (10mm for the plastic connector, ~10mm for the cable to bend). That means the non-Ti needs about 160mm in total, while the Ti needs only 150mm thanks to the connector recess on the PCB.
So the question is, where to draw the line? The M5 currently is 180mm wide, which is 20mm more than the M1,
however the M5 will likely use slightly thicker panels (2mm vs. 1.5mm on the M1), and a window attached to the inside of the panel will take up some room as well (3-4mm). And then there's the cable space behind the motherboard I mentioned... if I only add 5mm there, then the limit for the video card would be about 150mm. Enough for the 1080 Ti Gaming X, but not the regular 1080 Gaming X.
Well, talking about trends... gpus are getting bigger and bigger as time goes by, and I do not expect that trend to change. At the same time, we are getting mini versions of pretty much everything. So, as an extremist I am I would...
a) Design for 310mm long and extra height gpus
or
b) Design for mini-variants and screw the world.
I wouldn't even bother with anything in between, to be honest with you. Accept it all or accept only the mini versions. This way there aren't concessions to make nor "will this fit". The answer is pretty straight forward and everybody knows where they stand. A case that accepts 285mm will get a thousand questions on whether "this will fit that but won't" and is a pain in the ass as tomorrow the standard for reference gpus will change and you are fucked.
Just look at the GTX1080, for starters:
The MSI GAMING/ARMOR/LIGHTNING, EVGA FTW, ASUS STRIX, GIGABYTE AORUS, ZOTAC EXTREME and some others feature this extra-height pcb. 5 years ago? You needed the most extreme of versions to get it... but it is getting more and more common. It is a trend to stay. And for good reason: 99% of people have big enough cases to support them... and the bigger the gpu, the bigger the heastink, and the better the relation with noise/heat/overclock. I know how good my 1080 STRIX is: heck the fan doesn't spin unless I'm gaming (on my 50L case, that is) which is a HUGE BONUS when you want a computer that isn't screaming all the time.
So, I'd go all or nothing here. Either force people to get the mini versions or let them choose freely gpu-wise.
Regarding storage: I'd also ditch completely 3.5" storage. Nowadays 2 x 2.5" + M.2 + external storage should suffice for any use except for a NAS or SERVER.. and for such usage a convertible gpu-space should do (IE, trading the gpu space for HDD space). Again, look at your target market: a case that costs $50 is one thing, but one that costs $250 is another. If you have the money to afford a $250 case you can go pretty much all-SSD at this time, specially if it makes the case more compact and, thus, more likeable from the standpoint of something with the intention to build a high-end SFF computer.
In any case... why did the M1 case succeed? That is what you should carefully analyze. IMO, It sold because:
-It is a very beatufil design.
-It is a very beatufil design.
-Did I say it is a very beatufil design?
-It is build in a way that gives emphasis to the design as the materials and craftmanship are top notch (though the outer panels need to be thicker as they end up being bent on usage).
-It is a very flexible case.
-You can stuff pretty much anything there. Though some parts would bend a bit if you were using hdds on the side panel.
So, a good design sells. And if the good design happens to be useful, it sells more. But the design needs to be pretty in its own way. For instance, the SENTRY design is very unique because it combines a very useful-oriented design with a useful and rugged-oriented construction. It works as the design language is coherent. It is a very different animal than the M1, for sure, but coherency is what makes something work.
TL;DR: forget fashion-based market trends and look at what inspires you. BUT remember that ATX market is very crowded, and thus smaller cases get more public attention as good cases in the sub-25L space are non-existant. At this time, personally, what tickles my fancy is something like the SENTRY, IE a vertical case with the gpu on top of the motherboard but with room for fans so that you don't end up with a screamer or toaster and constructed in aluminum. With a pcb-based riser it should be doable without spending a fortune. And I bet it would sell because the SENTRY is alone in that market segment. We have the same half-assed cases we had 4 years ago. Nothing has changed there.
PS: and do not try to be over-perfectionist. The M1 got better with each revision, which isn't a problem at all. Sure, if the starter edition is flawed then yeah, but don't expect perfection as we live in an imperfect world. Heck I'm sure the LRPC, or S1, is still viable with a few modifications, specially nowadays when the 1080 and 1070 parts have lower power consumption than the last few gens.
