Yes, derivations are the main primitive to target in an agnostic way

Main challenge is the value of the nixpkgs repo is enormous, and that looks tightly tied to Nix the language and its implicit constructs. I think instead of an FDI one would have to provide a true competitor that is more attractive on multiple dimensions like:

- reworking Nix as more a modular architecture (see Tvix: https://tvl.fyi/blog/rewriting-nix)

- Tackling a package repo and Nixos equivalent with day 1 reproducibility (not just repeatability) like https://reproducible-builds.org/

- Better UX experience on the "entrypoints" of Nix like home-manager and dev-shell flakes - I think CUE has some nicer language features for this and does not require figuring out derivation generation, just referencing the existing nixpkgs set