The great thing about fanfiction is that it’s Fan. Fiction.
You wanna make it non-canon compliant? Do it, boo. It’s your story. Write the way you wished it had gone. Write the characters making a different choice. Fix the universe where you think it failed. Follow the guidelines or don’t. This fanfic is yours! Write and have fun with it.
You wanna make the story canon? With all the history and pairings perfect? You do it, darling. Figure out what happens after the main storyline is done. Figure out what would happen in that unseen/unwritten moment. Come up with a brilliant story for the characters and have them work it out with the history they’ve already been given. Write that fanfic the way you want to and have fun!
You wanna write an AU? You do just that, my dear. Write the characters in a world they’ve never been in. Use all your favorite tropes, or create something new. Have them act differently to things because it’s a new experience. Write a world you wished they’d lived in. Write your fic in whatever world you want because it’s your fanfic and you should have fun with it.
You wanna write a crossover? Freaking do it, babe. Have your characters meet characters from another story. Make them friends, enemies, roommates, or lovers. Make them all of the above. Combine two universes or set them up in one.
Write characters who are OOC, or write them in character. Write them in a way that fits your headcanons. Write them in a way that makes them come even more alive in your eyes. Write scenes that make your soul ache. That make you smile and laugh so hard you cry. That make you excited to keep coming back to these characters.
Too many people out there will criticize and judge you on how you write your story. But it’s yours. Not theirs. It’s your Fan. Fiction. You are the fan creating the masterpiece. Nobody is allowed to tell you what you can or can’t write for your story.
Tldr: Write the story you want to write. Have fun. Fan Fiction is for you to write the story you wish had happened.
THIS.