I know a lot of people have a lot of feelings about this and fanfic generally gets frowned upon in literary circles and people talk about how it can "ruin" you and all this jazz. Like, no. Listen, kids. You wanna write your fanfics, write your fanfics.
When people start learning to draw they copy pictures they already like. They learn to draw by doodling images of Mickey Mouse in their school journals and trying to figure out the lines that make up Goku. They learn to cook by saying "I like Mac and Cheese now how do I make some for me". They learn to knit by following easy patterns and eventually get good enough to freestyle something new, but guess what. Following those patterns is part of the learning process.
Follow the patterns of characters you already love. Write your side stories. Learn what makes them tick and what makes them work for you. Learn what parts of the story you love and why so when you go to create something for yourself you have a solid understanding of why that thing worked for you in the first place and how you can use it to create a similar vibe in others.
What you do for practice is not a waste of your time! Anything that is a learning experience is a great opportunity for you! So it's not marketable. What kind of capitalist drek attitude is that anyway? I mean yeah eventually you're gonna have to put it down and make something that can make you money, if this is something you want to do for your career, and that's fine.
But listen to me for a second, alright? Even if you're a 5 star chef that makes complicated seafood dishes for high powered patrons...sometimes you still want to make and eat some ramen for you. Simplicity doesn't make it bad.
Because it's its own art form and it's valid. Because if you're ever going to write for television you're going to have to flex those muscles and get used to writing other people's characters. Because no running series has one writer make all those episodes, they bring in other people, and sometimes you are gonna have to get used to keeping characters consistent when you did not create them and that is a valuable and marketable (-pulls face-) skillset.
Because sometimes you just have to take a step back and write something because you love the process of it and sometimes the easiest way to do that is to write a damn fanfic. You're not being childish. You're not wasting time. You're not 'cheating'. You're learning, you're practicing, you're playing and enjoying yourself.
So go write your fanfiction.
And then, when you feel comfortable, if you want to? Write something new, too.