I had a look over the various specs and blogs that people wrote about experiences with Blackbird and Talos II boards.
Here are some observations, maybe a wish list for future board designs:
Blackbird only supports 2 DIMMs / 2 memory channels, but for the price of these boards and CPUs I feel that it is very compelling to have 4 channels, as in the Talos II Lite, which is only a little bit more expensive than Blackbird anyway.
With only two PCIe slots on the Blackbird and Talos II Lite and no support for PCIe bifurcation, it is difficult for people to do NVMe RAID. It would be really useful to have a couple of M.2 slots on the board so that people can have up to two NVMe SSDs and still keep the PCIe slots free.
4K monitors are much more common today than they were when POWER9 was launched. It would be nice to have basic 2D support for a 4K display on the built-in HDMI port. From practical experience running Debian and GNOME, I found that small 4K graphics cards, like the NVIDIA K420 with 2GB were unstable but cards with 4GB or more have been trouble-free for a development desktop.
On size, it could be bigger than Blackboard but it really needs to be smaller than Talos II (Lite) to open up a wider choice of cases.
A board with all of the above would probably be somewhere between Blackbird and Talos II Lite.