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Dingasoid (I know it's kinda old but I hope you don't mind getting a reply!)
If you mean fandom characters as in; inspired and made to fit in the world of a franchise you love. Has anyone seen lawsuits of characters "crossing the line"? There was an app where yoshi was used in, lawsuit came, the only few changes that were made was making yoshi red and giving it a blue baseball cap. Then it was legally approved, and of course, taking out all mentions and tags of Nintendo, Mario, Yoshi, etc. You're fine! And most franchises actually won't care, unless you become a bigshot and try to profit. And then still, the changes are likely need to be minimal to avoid copyright infringement, whole discussion we could have about that. But you're not gonna get sued over a silly old RP on the internet! Most companies welcome fan creation and support it, it's free advertisement!
If you mean fandom characters as in; RPing the character from Canon. As an example, Spider-Man. No, not wrong at all! But I'll admit, there'll be some real blurry lines. For big franchises, at least 30+ people have written for that character legally. Legally, not allowed for you to write about them, but realistically, no one minds it. If something back in your head feels anxious or disrespectful, so many different people have written for that character. You've got your own special view of them you should be allowed to write! It doesn't really belong to anyone but a faceless company, so many people have written for that character, you're not offending anyone with it.
Though with indie projects, it doesn't hurt to ask if the creator is fine with what you're doing. Like Stardew Valley, mostly made by one dude, but he has said many times he's fine with all the creative works people made. But someone on a much smaller scale may not be comfortable with that, the owner of that character hasn't become ambiguous like many pop-charas.
(And honestly, most people don't even credit their sources of franchise media they use, so it's cool enough you already do! And it's cool of ya to not take fanart without permission and credit, I've seen that enough, as an artist, makes me uncomfortable. And what you do isn't making me uncomfortable if that brings you any peace)
Main point I wanna make, nothing is wrong with innocent fun! So enjoy yourself, you ain't hurting anyone! You're not the first doing this, and you won't be the last.
So don't worry. Also copyright law, especially USA, is very outdated by 200 to 300 years and doesn't really apply to online shenanigans. Especially if you're not making money off it or trying to deceive people in anyway with it. So, have fun!