Many authors I've met recently during events have started suggesting, ever so tentatively, that fanfiction can be a good way to flex writing muscles.
I, for one, would like to climb to the roof of my apartment building and shout that to every young writer in Brooklyn (and to all of you, dear friends, who have decided to absorb the words I send into the ether). I know that even as recently as when I was in high school and college, fanfiction was a secret word saved for your carefully curated Tumblr--people never discussed it, and teachers rarely acknowledged it. But, if you give it a chance, I think there's a lot of value to using fanfiction as a writing exercise.
Just as many visual artists talk about the first steps into developing your own style (tracing and imitation of artists you admire is something I've seen endorsed quite a bit for learning purposes), I think fanfiction serves that function for authors. In my experience, too, it's a great way to quiet the imposter syndrome and turn off editor brain for a bit. It can also help spark inspiration if you've got writer's block. When you're playing in already-established IPs, there's less pressure and more wiggle-room for off the wall ideas.
If you want to work on worldbuilding, take characters you know well and bring them into an unfamiliar environment.
To work on dialogue, you can put two characters into a situation and see how you interpret their reactions.
To write a specific genre (or a common trope in a genre), you can just as easily put other characters into that situation and allow the scene to play out.
I could certainly go on and on about why I think fanfiction is so easily dismissed by many, but that is a soapbox for another day. For now, I would like to leave you with a small challenge:
Find an aspect of your writing you would like to improve upon, pick a show/movie/etc you know intimately, and just write. If you've got a case of writer's block, exploring new relationships with characters you're already familiar with is a great way to keep the creativity going while giving your brain a rest. Be cringey, write that coffeeshop AU, make the crossover you've always wondered about. Let the story take charge and see how that affects your other writing.
Moth Detlaf (they/them) is a bookseller and the social media manager for Books of Wonder in NYC. They can often be found huddled in a corner somewhere surrounded by a palace of queer books for all ages--bonus points if there are dragons involved. Finding the books that make kids of all ages and demographics (including the grown up ones) feel the way they have always felt as a reader is one of their favorite things.