The Nix package manager is brilliant, it works on all (most?) linux distributions and macOS and it even has more packages than AUR (see https://repology.org/repositories/statistics/total). It would be great if there was a nix backend for discover.
This would be something to implement in PackageKit, not Discover directly. PackageKit is the layer in the stack where support for different packaging systems gets implemented. See the current set of supported backends here: https://github.com/PackageKit/PackageKit/tree/main/backends