GNU Guix provides state-of-the-art package management features such as transactional upgrades and roll-backs, reproducible build environments, unprivileged package management, and per-user profiles. It uses low-level mechanisms from the Nix package manager, but packages are defined as native Guile modules, using extensions to the Scheme language—which makes it nicely hackable.
I am extremely out of the loop, but I believe until somewhat recently it still used nix-daemon and the corresponding drv format, but it may have since been rewritten.