Good afternoon, Mr. Wheeler. I’m calling from the gas company. I hope you’re doing well and spending these final hours in peace with your loved ones. Rest assured that we will make every effort to keep the gas flowing right until the end.

• Capturing a character’s essence.  Nailing a character’s portrayal is tough to begin with, but accurately portraying someone else’s characters is even harder. Mastering someone else’s characters is a great way to learn how to keep your own characters consistent and shape their personalities believably.
• How to write something you’ll enjoy and feel passionate about. What better way to start doing that than to write about characters you already know and have grown to care about. Thinking about the kinds of stories you’d tell with other people’s characters will help you figure out the types of characters and tropes that appeal to you.
• Critically evaluating causal relationships. One simple change to canon can make a big difference. Exploring how changes to the plot affect characters is a great way to practice looking at the big picture and considering multiple paths for your plots to take.
• Taking inspiration from existing settings. Having an existing setting ready to write about (if you choose to stick to a setting from canon) lessens the burden of world building and thereby encourages deeper thinking about the finer details instead of larger elements. Things the creator didn’t have the time or desire to address are yours to play around with. This will help you get a sense for things that feel like they fit well with the established setting. It will also encourage you to look to real world settings for inspiration too such as cities in other countries. Once you’re more comfortable exploring fictional settings, you’ll be more keen to dive into real ones too.
• Letting your imagination run wild. You might initially think working with a set cast of characters and/or world might restrict your creativity, but, much like restrictions in original writing such as rules about what magic can’t do, these restrictions encourage creativity. The characters and/or setting may belong to someone else, but your take on them is your own.
• How to handle criticism. Commenters can be very nitpick when it comes to franchises they love. As you receive feedback, you will learn what kinds of feedback are useful and which you can disregard. You’ll also learn how to deal with critics and network diplomatically. PR is important for published authors, so getting some practice with fanfiction commenters is a useful learning experience.
•  How to research. Double checking details about characters backstories or details about the setting isn’t that different from researching real world things.
• Basically any other skill you can hone by writing original fiction. Practice makes perfect. The key to improving your writing is to write whether that means writing fanfiction or completely original works. Neither original fiction nor fanfiction is incapable of teaching you a wide range of things about topics you might not have cared about beforehand.