This idea is driven off of two remarks made by Dr. Upton plus the failure of a product that came close, but didn't really get it right.
Background
Dr. Upton said (in an interview I read) that the RPT only wants to make products (other than Pis) that marketplace is either not providing or the items being built didn't get it right. He has also said (recently) that the RPT is looking at making a CM4.
WD Labs produced a "SATA Adapter" board, initially for use with the CM1, and later--as it turned out possible (sometimes)--for the CM3 boards. There turned out to be a couple of problems with this otherwise interesting carrier board. The first is that the adapter has only 2 USB connectors and no Ethernet connector, though it does have an HDMI connector. Since all of those are the "full size" versions, and all are along one edge (which, given the packaging and mounting does make sense), that's all there is room for. (And, I should note, the USB connectors are not stacked.) The second flaw is that, in my experience, the early boards fully supported booting directly from an attached SATA device (CM3L only, of course), later boards couldn't do that. I suspect an incompatibility in the SATA bridge, a problem that has been found in the wild with USB to SATA adapters.
Proposal
Therefore, I think that once the RPT launches a CM4, I would dearly love to see them build a *proper* CM4 SATA Adapter board. It should have at least 2 USB ports (USB 2.0 would be fine), Ethernet jack, and one HDMI output would be sufficient. It might be interesting to use the single PCIe lane from the SoC to be the mass storage interface, either as SATA or as single lane M.2. The common connectors--USB, HDMI--could be micro for all of me. It would be nice to retain one other feature from the WD board...an actual power button.
Note that if such a thing came to be, I would not expect to see it for two to three years....or more. If, in the mean time (once a CM4 goes into production) someone else wanted to step up and market such a device, go for it.
Discuss.