I can imagine that it’s still a little bit confusing to imagine this board as a shield for Arduino, as it can act perfectly by itself.
This open the possibility of this FPGA Board to become a series of boards, like those Sipeed boards - e.g. - An independent/solo board, a shield/hat version, etc…
One suggestion that I can see for this little guy for now is to expose more pins!
If the intention is to have it “also” as an Arduino Shield, I think it would be excellent to expose both the Arduino pins (as does the shields, normally), and also expose pins of the “accelerator” shield itself.
Think of it like this shield, that appeared on Hackaday:
See the “Additional I/O FPGA” lines in the middle of the picture in the right, along with the Arduino headers on the extremities.
And, a final suggestion, could you think in a Raspberry Pi HAT in the same way? I think it would be even more useful as the “edge accelerator” that this board intend to be!