You have it more or less covered, not sure why it took so long for someone to use a PCI riser to horisontaly position the gpu but as far as I know there are no mainstream cases for this configuration.
I looked into this a few months ago when I was looking to build an HTPC with a high end gpu. Node 304 or sugo 05 is as good as it gets. I don't consider them that big and I prefer the cube form to the nettop look. Unfortunately with these cases you need a gpu with a reference cooling solution(rear exhaust) and I already had one with a tripple fan config.
If you are in for a high end desktop system then the premium for the Bolt is almost reasonable.
In theory if I wanted to do this myself, I would probably start with a small mini itx case like the m350 in your image, an external psu, the gpu outside the case with a flexible PCI riser and then mount both of them beside eachother behind the monitor. It probably wouldn't turn out pretty. Then there's the perfomance hit due the riser, which there is very little information about.
I would love to see an all-in-one water cooling solution with the cpu, mobo and gpu sharing a common block (gpu horisontally positioned on top of the block), or even better a mobo with an intergrated high end gpu and a common block with all other connectors on the rear of the mobo. If all that ever came true it would still be powered by a brick sized psu(zero form factor progress in 30 years).
