If there is somebody willing and available to write an RFC about cherry-picking parts of HM in Nixpkgs then I’d be happy to assist in what little way I can. For example, the dagOf type would probably be handy in a few places in NixOS.
In general, though, I’m inclined to agree with @edolstra and don’t see much point in having HM as a whole merged into Nixpkgs. Perhaps the main benefit would be having a pre-build copy of the HM manual in the binary cache.
What I would be more excited about is an RFC for some form of library of stable “options only” modules. The idea being that different projects could import and endow them with a project specific config section. For example, it would be nice if I could more or less copy the services.postgresql attribute set from my NixOS configuration into my HM configuration to have my own PostgreSQL server, or into a project shell.nix to be started up by direnv, or into a single service Docker setup running without systemd, and so on.