There isn't a single one, but off the top of my head, the following package managers have been used to meet the same need (building Linux distros) but use distinct and relatively new paradigms or techniques:

  - Guix, Nix
  - Distri
  - Luet

(I'm personally partial to the 'functional' school, which means Nix and Guix. :)

One might also count package managers that are designed for building large, multi-language distributions of software but have never been used to build a whole distro. In that case, you could also include

  - Spack
  - Denxi

and finally you might also include tools that build on top of the traditional stuff, like `rpm-ostree`. This is what you're dealing with on Fedora Silverblue, SteamOS, openSUSE MicroOS, etc.

Also distinct from these but not particularly new are source-based package management systems, like

  - source-based Linux distro package managers incl.
    - emerge
    - paludis
    - sorcery
  - various ports systems, incl.
    - pkgsrc
    - Ravenports
    - FreeBSD Ports
    - Dragonfly BSD's DPorts
    - MacPorts
    - Homebrew

but those aren't really in the mainstream of system-wide package management, at least from my Linux-centric perspective, so I don't usually mean them when I refer to 'traditional' or 'conventional' package managers without any other qualifiers.

Anyway yeah, plain old openSUSE and Fedora are great choices if you don't want a declarative system or immutable Linux, and that is in part because zypper and dnf are best-in-class for their paradigm.