@Necere
I have some ideas that I think you should consider for this project:
1) Design around E-ATX form factor in order to provide wide CPU/motherboard support.
2) Focus design on single GPU (up tp three slots) and WC, with dual GPU support only for single-slot water-blocked cards.
3) Keep pushing for SFX(-L) PSU's. It's just a matter of time 1KW SFX-L PSUs.
4) Keep pushing for space efficiency (goes along with #3 and #5).
5) Consider using PCI-E riser(s) to install the GPU(s) above the motherboard (PCI-slot region), basically rotating the case PCI slots 90 degrees. Support for 3 PCI slots. This enables a slimmer case profile and...
6) A 240mm radiator+fan unit can be installed above the CPU/IO region of the motherboard (ala M1). This saves from the height of the case and...
7) With the SFX(-L) PSU under the motherboard, the case dimensions would be around 400 (H) x 320 (D) x 140 (W) = 18 litres.
8) Additional (if really desired) 240mm rad+fan unit support can be provided (on top of the motherboard) by increasing the case height to 450mm with a volume of 20 litres.
9) Other things covered by this idea:
a) The area in front of the PSU can be used either for a 120mm fan, or up to two 3.5" drives, or a low profile pump (for WC).
b) The front panel can be perforated for adding some fans there, or be made solid to install 2.5" drives on the inner side.
Let me know if words don't suffice to describe the idea and I'll put together a mock-up