NixOS is a distribution built around the package manger Nix. Nix is not necessarily an iteration of Flatpak ( especially since it’s been around since 2004), but it does accomplish many of the same goals in a more robust way with fewer trade offs.

The main idea of nix is that EVERY dependency of a package is tracked, from the exact glibc version all the way up to e.g. Python packages. I am not a Nix expert, but my surface-level understanding is that this is accomplished by hashing the package and all its dependencies, very aggressively, so that even if a hot fix patch is released that doesn’t change the version number, the new package is still different (as is every package that depends on the new version). That enables Nix to be the best of all worlds as far as sharing system packages like a native dependency while assuring stability and encapsulation like a flatpak. So it ends being as fast and small as the former while being as convenient and cross-distro as the latter. There are other innovations, like declarative dependency management and perfect rollbacks, that make Nix/NixOS stand out, but the above is it's main innovation over Flatpak and older system package managers.