nixpkgs and cppnix forks should stay MIT imo
i feel it would be extremely unfair to not just the maintainers, but the entire community of either project to take their work and license it in such a way that they can no longer realistically use it. i would like to see a future where we can go our own ways, but still improve each other as projects - especially with a member of the nix team already reaching out. a copyleft license could easily create issues here