One of the fun aspects of fanfic is being able to write in a different point of view than the canon protagonist. There are a couple different common ways to do this. Supporting character, background character, or a complete outsider POV. (That last is usually an OC.)
Outsider POV can be one of the most fascinating ways to pull this off. How does a complete stranger see your heroes, sometimes without ever knowing they’re heroes at all? You can run the gamut of reactions from “Wow, that was amazing!” to “eep, those guys are scary,” to “Really? Really? Why?” to “what just happened?” Just take the canon setting and a typical series of events in it, and consider what they’d look like from a hapless bystander/lawyer/postal delivery guy’s POV. Instead of an adoring audience safely on the other side of a TV screen.
Be warned, though. If you want to pull off outsider POV, you need to know canon well enough to pull in hints and references that will be obvious to a fellow fan. In essence, outsider POV leans heavily on being a series of in-jokes for the reader. One of the most effective fics I’ve run across was a Stargate fic, where a photo lab employee ended up developing pictures from a camera owned by a blonde Air Force major. Let’s just say, that camera really got around. (From a real life POV, security breach! For fic, hilarious.)
(I’ve looked and can’t currently track down the fic. I think it was on the Heliopolis archive? Drat.)
If you’re not sure you can toss in a scatter of canon references and have them make sense without the characters themselves, you might try for supporting character or background character POV. Supporting characters are better when you want to get close to the protagonist and canon events, but look at them from a different angle, with details and interpretations the main character might never see. It’s fun, for example, to read BnHA fics in which Eraserhead asks just what Nezu and All Might are thinking, setting up mock combat scenarios that pit a bullied student against the one who still wants to blow bits off him.
(I’m also in favor of canon divergence fics in which Bakugou finally gets shoved into anger management sessions. Or the even rarer fic in which Katsuki figures it out himself and starts working to be better. See Viridian: the Green Guide for that.)
Background character POVs split the difference between outsider and supporting. They’re a known, if not always named, character in canon, so you have a starting framework for why they’d be mixed up with the heroes in the first place and what they might be like. But there’s usually plenty of room to add details and background to them so you can make a more rounded story character. I recall the poor low-ranking Imperial officer busily working at his keyboard when Darth Vader “accepts an apology” from an Admiral in The Empire Strikes Back. Hears the thud, looks behind him to see the body, up from that to see Vader on the screen-
Turns swiftly back to his keyboard, and starts typing again. That would make an excellent basis for writing a fic about what the average Imperial thinks of fighting the Rebels, serving under Vader, and everything else.
Fanfic is a way to have fun by getting into more of what you loved about a fandom. To poke all the corners and details the creators might not have had time to flesh out. Try poking different POVs, too.