I have been thinking about picking up this miniITX, Cooler Master NR200P[0]. Similar form factor that would be completely adequate for a daily driver and could accommodate a sizeable GPU if required. The problem is that the smaller builds are still niche so you have to pay a premium on an appropriate motherboard and power supply.
Yes, there is a standard called PICMG used in “portable/luggable computer” industry - oscilloscope-shaped PCs for field uses. Kontron and Advantech seems to be major suppliers there.
Is there a case like this, but that is not part of a prebuild computer?