I am not sure rayon is the kind of thing that should go into the std.
Libraries like crossbeam are probably more appropriate for the std. Lock-free containers(data structures). Most containers are appropriate for inclusion in the std 
If a library is to opinionated it might not be good for the std, although, I don't think rayon is very opinionated on how things should be done(unless the work-stealing implementation is considered opinionated - I don't think the implementation can be swapped at the moment, although it is configurable).
I think the current implementation of the HashMap(in the std) is more opinionated since it decides by default to be oriented on security, although most users might not need that.