If we do a backplane, it'd be be basically a PCB and edge connector version of PCIe cable + USB cable. The only thing a backplane might do differently is integrated USB hub chips so there's only one cabled connection between the backplane and a controller. All our boards will have USB connectivity, so making a box with a bunch of 'em in it would be possible by sticking a several-port hub in there and getting a bunch of short cables.
That is the main idea, simplify cabling and parts with a pluggable backplane, and replace boards as need arises. It currently is a shame that every new iteration of a chip or a miner design you have to get new controllers, with all the waste associated. With a hotpluggable design it is just a matter of remove board, plug new board, and hash away. No downtime and no extra waste. A board fails, boom remove it from the board and replace with another one.
Also, you could have several different cooling solutions, with standard air cooling as a default.
Anyways, these are exciting times for this project.