Think of UI design tools like Figma, but having their own frontend UI framework like VueJS or React, coupled with an optional OS abstraction layer like Electron. The official UI of the design tool could be the base for the UI of the OS.
GTK started similarly, as Gimp Tool Kit. Only that the base was kinda flawed and not really geared towards professional interface design, and it couldn't aquire credibility as design tool because of that. And if no professional designers use your tools it will be hard to get professional designers to contribute. That's the dilemma of GTK and Gimp.
Base something like GTK on a dedicated interface design tool (like Figma) and other tools that complete the designer, developer experience, and this could totally rock.
I think that a popular design tool that integrates a frontend UI framework and abstraction layer for the backend/OS, would also attract frontend and backend devs automatically.
The main flaw in todays linux environments is the lack of focus on professional niches outside of devops and sys admin tasks, especially professional designers. I wondering about what could be possible, if companies like Figma and Mozilla would cooperate on a design centric distro..