Another thought: Gentoo has a rather complicated "Atom" syntax to convey
which parameter combination is acceptable or not.
I found it very hard to use at times.

With Guix, I believe we can do better.  We can add a
#:parameter-predicate field which takes the user parameters and returns
a boolean (pass or no pass).  This would give us maximal flexibility.

-- 
Pierre Neidhardt
https://ambrevar.xyz/