>what OS language(s)
OS: SeL4 with BSD userspace and some ports-like software management system
Language: Doesn't really matter, but if I would have to choose... C
>what architecture/ISA (turtles all the way down?)
RISC-V
>other requirements/features/quirks
Completely open-source, preferably permissive license. Including hardware security measures. Also, powerful enough to at least to be able to play Portal 2 on it in minimum 45 FPS.