WYSIWYG
This could be something very simple, like https://facebook.github.io/draft-js/, and could evolve into something very complex, like webflow.com.
Let's say you have something like Material UI react components. They have nice documentation, but what if you could drag and drop various components in order to nest them, and make composite components.
Add support for columns, some rich text editing, and bam, you're pretty close to mocking up entire web apps with an in browser UI.
And, it could be used to create simple content, like writing posts. If you've used weebly, SquareSpace, or a nice WordPress theme, you might have an idea what I'm talking about.
Content entry is still one of the holy grails of the web. Proof: why do so many sites use markdown?