Where Nickel fits to this? Will it replace Nix or co-exist?

"Nix" refers to many things. Nickel specifically replaces the language that is called Nix. It has nothing to do with the package manager called Nix, the build system called Nix, or the OS called NixOS.

So, will it replace the language called Nix or co-exist with it? How will Nix package manager and build system handle this new language?