Glad that 8th slot is getting recognized! I had a ton of people who do GPU builds ask for that. Can't believe so many other companies over looked doing that, particularly these days.
No kidding! And thank you for the 8-slots! Here's an idea to keep in mind with Epyc/Rome server-type builds: there is no chipset. This means we are basically free to install the GPU in any slot we see fit. For servers, this is especially nice to precisely control the air flow throughout the chassis, fans, M.2, NVMe drives, etc. Sure, you can do this for Intel and other AMD chipsets: but they have to be particular about which exact slot they can use.
I mention this because I have an idea that I haven't seen any other case designer do. And the fact that you are thinking like that, I'll give you this idea to consider. Always wanted to design a chassis myself...
The idea is about inserting other system components into that GPU area, that would normally clash in those PCIe slots next to the CPU (you need 2x or 3x slot widths for GPUs!!). Think, 16 or 20 drive bays. Think, water cooling component locations, rads, etc.
The idea is that the builder could move the GPU, say, to the very side/top/bottom of the chassis - into the last slot. Then you have all of that room between the GPU and CPU cooling area (which is pretty low with AIOs).
Obviously, that's a very small use case. But, maybe... maybe something modular.
I am still working on the NAS cases. That drive requirement - would that need to be front loading, or could it be top loading?
I am really thinking that something like the 45 drives cases for DIY builders is something I am going to do. It's also the only way I see to do that many drives with any real density.
NAS 4U for a rack? To me, it doesn't matter as homelabber. You tell the family, "hey, no one can watch TV for a while", shutdown, and just pull the beast out the rack.
To let ya know a few existing designs: A front-loader that has pretty-much maxed out all capacity you get get would be the Supermicro SC846 at 24-bays. That's, 24x3.5" in the front fills the entire front (and rear) areas.
The Supermicro SC847 is a much more interesting chassis to study though, especially if one was to use a 2U riser to mount a GPU sideways?! Search for pics of 36-bay AND 45-bay variants. NOTE: the 36-bay variant only has room for 1/2-height PCIe cards and CPU coolers. Whereas the 45-bay variant has zero room for any motherboard what-so-ever.
As for connectivity, you don't need to have a backplane for "Front-Loader" types like this. The drives can slip in and have their asses exposed for connections.
If you are open to playing with different form factors, you could investigate cramming an mATX or ITX into a 36, 45, etc-bay build! That would make for a fairly compact design.
Now, for Top-Loaders... In top loaders, I've seen 45, 60, and even 90-bay systems. However, I think that would be a fairly difficult design, mainly because of the tight-clearance from placing a drive vertically. This is why manufactures like Supermicro and HP have created very large PCB "backplanes" that are nothing more than a 90-degree right-angle of power and SAS connector (and now even U.2!) of where the drive is inserted.
That is, unless we can find an endless supply of dirt cheap PCB SAS-connector backplanes?
Also, about the max you can cram vertically and have plenty of air-flow is 15x 3.5" drives, side-by-side. So each row is 14 or 15 drives (some top loaders use 14-bay rows). if you want some inspirations, look at the Supermicro 60-bay as it has a little room for an ATX if you design your own chassis (90-bay chassis cannot fit any motherboard!).
The Cisco C3260 is another interesting chassis to study as a top-loader. You can think of replacing the rear two nodes with, say, an mATX or ITX board.
Good luck!