This thread reminded me of something I tried a couple of years ago: the MTX538.
The distance between the rails on the MTX case is exactly Euro Card PCB size, So I tried squeezing a much of an MTX a possible onto a single Eurocard and came up with this:
Power would have been a 7.5v 1A supply driving a single 5v regulator positioned to use the existing holes in the MTX case.
There would have been a V9938 on a daughter card sitting over the 2, 64k x4 bit drams. The output is a 20 pin header of the same format as the one on the MTX plus except it's driven by transistors and not a CMX1645 to save space.
Memory wise there's provision for a 128k Flash containing the system roms but only a 32k 0.3" ram (also to save space).
For I/O I managed to squeeze in the keyboard, CTC, sound chip and an IDE interface. Memory and I/O decoding needed 2 22V10 GALs.
If I were trying that now, I'd probably try using one 84 pin CPLD and try to ditch the GALs and 74 series logic.
Physically it just about fitted under the keyboard, though the clearance for the CPU and CTC was very tight. It might have been possible to scrunch the chip sockets even tighter to fit a 128k 0.6" wide SRAM. However the wiring was already pretty awkward with the existing spacing.
After issues with the 9938 and Bill coming up with the propeller 80 column card at Memofest 2016, I stopped work and build the MTXmini that Mark photographed at Memofest 2017. viewtopic.php?f=25&t=365
That used a card and a half to fit everything in. But was MUCH easier to wire and has a larger RAM chip required for CPM.
A double eurocard design (so 200mm x 160mm) would be able to accomodate pretty much everything and still only be 2/3 of the size of the production mainboards. Though I don't imagine hand wiring one would be too popular
I think there's a photo of the part populated MTX538 board in Andy's Memofest 2017 photos.