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I'm sure prolewiki would love to host several gigabytes of leftist fanfic, let's do it! 
in all seriousness though, why does the site need to be leftist? You are free to post it here, ofc, but I just don't see the appeal of a leftist ao3 clone really...
also I meant to specify "original" works.
Well I'm clearly an idiot.
nah you're not, I'm just not a writer or anything so idk of a place like that. [email protected] might work tho
I don't think lemmy is capable of long-form text (multiple pages).
hmm true there are limits, and continuing in the comments isn't exactly elegant.
There;s something called AO3 but all the fiction there is about getting your cis werewolf boyfriend preggers.
its called fanfiction.net there's literally a 190 chapter fanfiction called Harry Potter becomes a Communist what more do you people want
spoiler
its a bit but there's a character called Halliburton Pepsi so that makes up for it
Just throw it on Wattpad