currently about how nixos would be a good base to build an immutable user friendly distro on sounds insane at first but i reckon a mechanism for automatically grabbing and building a signed system config would get me 70% of the way there would only need to make all of the supporting software
intended workflow for normal users would be flatpak and appimages, while people good at computer would be highly encouraged to use nix itself for managing the software installed for their user
i had this idea myself before and i think it could go really well. a kind of pain point i foresee though is bloated initrds - by default nixos-install generates a hardware configuration file with the modules that should be needed to boot (graphics, storage, etc)
i don't see the initrd being a big issue all that's really needed is enough drivers to mount the actual rootfs