@thebookharvester, It’s cool to know that someone else started with fanfiction too! When I was twelve I started writing fanfiction for the Avengers (at the time I didn’t know the word fanfiction so I called it “piggyback writing”). I created my own characters who lived in the “shadows” of the Avengers (all my characters were either sons/daughters or nieces/nephews, sisters/brothers). After a few years, I went on to write my own original trilogy. I have dabbled in anything from Marvel to Star Wars, Star Trek and even Divergent. I created A TON of characters for Star Trek. With Divergent, I HATED the ending so I rewrote it and that opened the door for more stories (most stay in my head though because…reasons).
As to your question of practice, I think it is good to use because there is something already there and you can just add or change. I found that it was a good way to learn to “walk” as a writer. Learning to write using fanfiction helped me to figure out how to structure stories and how to be creative.
So my suggestion would use whatever fandom you like/enjoy. I also found that fanfiction is the way to insert yourself without actually doing it. You create a character like you or what you want to be like.
INFJ, Child of God, wannabe author, writer, dreamer, fan of DR. Who, Star Wars, NCIS-LA