Having good experience with a QM5000 board with embedded 15w APU, it would be nice with a barebone solution with maximum width for the GPUs, and very basic extras 8e.g. embedded CPU, DDR3 SODimm etc).

Placing Server PSU(s) on top of the CPU in this case would make it a compact design, but why are the 6-pin connectors sticking out at the bottom of the picture? I would think that would be a case wall in most designs? 19"= 48.26cm MB 48.5cm long ... I.e. it will not fit 19" cases/rigs, so the 6pins at the edge will be sticking out anyway.

Is the heat in these compact designs really a problem? How many "efficient" cards are running higher than 150W nowadays (about 1200W of heat to remove)?
I realize that it would be nice is the failure of one fan does not overheat the whole thing, meaning that a margin (10C?) to terminal failure is needed.
But my knowledge of cramped farms i null. 9x80W in a roomy wooden rig makes the cards run at well below 60C (ok, 19C ambient...).