It is. But I have this radical idea that with a declarative toolbox like what NixOS offers, most distros are trivial.

Lots of simple distros are doable with a page or two of declarative Nix configuration. They are just some opinionated choices concerning a variety of components, a desktop environment and some default applications (or the lack of thereof). Gentoo is also a meta-distro in some ways.

Of course, if you want some fundamental changes, you might not be able to stretch NixOS that far. Or if you prefer an imperative approach to things.

IMHO, most distro designs (imperative/declarative, binary/source-based) have been explored already. Forking yet a new distro is not advisable, unless you have some novel ideas or just want the educational experience.