I think it would be a good Idea to move theme packages to separate Categories defined in the Desktop Theme Specification.
So every Icon Theme would be under iconThemes.${pname}, every Cursor Theme would be under cursorThemes.${pname} and every theme package, that fits the Theme Package Specification would be under themes.${pname} or similar.
We could also make a wallpaper category, but wallpapers are kinda trivial, almost always part of a Desktop Environment, and thus in its separate category anyway (right?).
If we are at it, maybe also fonts?
Another advantage would also be easy discovery of available theme/font packages in the nixpkgs search.
Do you think that this is a good idea, that fits with the general goals of Nixpkgs?