I think this is a better approach than trying to come up with a definitive set of themes. The 'look' of your editor is a pretty personal preference and we are unlikely to find a definitive set of themes that will suit all. On the other hand, if we can make it easier to define a theme, we are enabling people to make it look the way they want, which seems to be part of the core essence of Emacs - do it your way.
At present, while you have all the power to customize faces, it isn't necessary easy to get things consistent. Defining a new theme usually takes a lot of trial and error. It has certainly improved since I first started with Emacs in the 90s, but could probably be improved further with a theme generator. The big challenge is that Emacs has far more 'faces' than other editors and it sin't always clear how all these faces interact. Some sort of theme generator which could help with this would make it easier for people to define their own themes.
The spacemacs setup also has a pretty straight-forward way to tweak existing themes with their theming-modifications list, which makes it easy to tweak specific settings for specific themes (and ensures they are applied when themes change or are re-loaded etc).