[sannee]

This shows a pet peeve of mine: the Raspberry Pi does not have USB available in a way which would allow a HAT to interface with it in a more sensible way than wrapping around a cable from the "external" ports.

I wonder if there is a pair of 90 degree USB connectors which would height-match a standard .1" header.

[topspin]

Perhaps the "Compute Module" is the thing to use for Raspberry Pi clusters. Probably fit 100+ in 1U with no cables at all.

[rbanffy]

I'd love to see a revision/extension of the Pi connector adding the Ethernet signalling. This would make it easy to build clusters using any board compatible with the ecosystem. The SOPINE (a Pine A64 cousin https://www.pine64.org/?page_id=1491) seems to have the Ethernet signals on the edge connector, making a cluster much easier to build out of a single backplane. The Rock64, an A64 successor, has the Ethernet on pins, but the position is a bit awkward.

But yes. At least the USB should be available on the expansion bus.