Depends on your specialization/focus in frontend development. What do you want to end up doing or do already and want to improve?
Do you work on UIs for apps, possibly web and mobile apps? It might be useful to pick up React Native, or any UI framework that allows for primitive - yet universal elements for web and mobile.
Do you create frontend modules that require input from the user? Or maybe you just want recyclable and more important, reliable components? Try experimenting with a testing framework to make assertions on your code and ensure it's working.
Do you create a lot of landing pages or microsites? You might want to look into static site generators (SSGs) like Gatsby, NextJS, or even Jekyll/Hugo/etc if you're into non-JS options. SSGs make the creation and optimization of websites much simpler than just writing HTML and CSS from scratch - and gives you performance benefits without having to figure out yet-another-library (Webpack).
Maybe you do a lot of repeated processes (minifying CSS/JS), or want to take advantage of stuff like PostCSS processing. You can get into Gulp, Grunt, or any other task runner.
There's a lot to do! I'd take a look at this chart on Github, it breaks down all the most of the paths for a frontend developer:
I'd just focus on getting really good at Javascript, since it's the basis of Node and React. Once you get a grip there, create apps and experiment with different tools, libraries, and frameworks and see what sticks into your workflow.
Hope that helps! 👍