I had an idea like this for a university design (Not really centered in design, more on design methodology and research) class. At first I thought of designing a fully customizable, plugin-based EMS (Like Moodle), then my teacher told me to try and make it bigger, so I designed an app platform where there's only extensions and the UI is fully yours, kinda like Notion Enhancer [1] or Better Discord [2], but without the base app.

[1] https://notion-enhancer.github.io/ [2] https://betterdiscord.app/