Roadmap?
A general overview of the planned features, refactoring plans and some way to keep track of who is working on what would be useful for new contributors.

Or at least, I would like to contribute whenever I have some free time, but I am not sure how should I implement things, or if what I am doing might conflict with someone else's work.

I have seen some projects use Github Projects or Discussions for that. Even using the Wiki might be useful.