Fedigrow

You'd obviously need to abide by the rules of the instance your community is on. Whatever those are, because that's the server it is hosted on. You could copy those, or be more strict. I'd add rules on how to flag stories as NSFW, so nobody gets weird stuff in their feeds if they don't want to see it. Plus something that makes it clear it's fictional works. Otherwise some Lemmy users are bound to get confused, once they watch the All feed. And I'd make it clear whether you want copyright violations, or if those should be original stories, written by OP themself.

Maybe have a look at other internet forums for fanfiction, dark fiction or whatever. There are lots of things to consider. Maybe people will hand in low quality stuff, ragebait, AI generated stories... You can make rules for all of that.

And maybe take care of your authors. Lemmy has downvotes and annoying people. And it's super toxic if you take several weeks to write a story, and other people hate on them.

So, you want to give your userbase creative freedom, without allowing it to attack itself or marginalised groups. Is this right? Here's what I'd suggest:

Rule: fanfics featuring prejudice must not show it in a positive light.

This rule alone would shoo 90% of the potential bigots, who'd likely cry about "muh freeze peaches", but still allow people to have bigotry in their fanfics.

Rule: fanfic discussion must be kept on-topic.

If there's something that bigots love to do, it's to pick on some loose end of a discussion, then pull and pull it until the discussion gravitates around their pet issue; this rule would make short work of this tactic. It's also useful to improve general quality of the discussions.

[If allow RP at all] Rule: Roleplay is only allowed in a specific subforum / community.

Some people actually like to RP their OCs; to each their own, I guess? Either way, if you let people RP all over the instance/forum/site some might use it to say "ackshyually I was RPing" to attack other members.

Don't feel afraid to change the rules as you go. You'll get a thousand problems that nobody could predict, it's fine to address them. As long as you do it transparently, it's fine.

A few additional points:

Keep a close watch on users who gravitate around certain specific topics. Most of them should be OK, but some might be looking for trouble. Look specially for signs of sealioning.

Recruit mods with good reading comprehension and who are OK reading huge walls of text. You'll need it for subtleties like distinguishing "the MC is racist and this is shown as a character flaw" versus "the MC is racist because the user is self-inserting".

Social media is full of a certain type of user full of "good intentions", who points fingers at other users based on weak reasons (such as poor reading, assumptions, etc.); for example, if you say "Hitler was shit but this has nothing to do with him eating bread!", they'll pick on that "but" and claim that you're a Nazi. Well, I call those "witch hunters". Don't let witch hunters in your community - they make people feel insecure to approach certain topics, and they indirectly help the ones whom they allegedly fight against.

I also feel like your idea leans more towards an old style forum than a Lemmy instance. So perhaps NodeBB is a better choice in your case.