Most fan fiction I see on wikis isn't identified as fan fiction by the person who made it, so it gets treated as a nuisance, false information or vandalism. Most people never show back up after they make it, which signals they didn't really care about it in the first place and that equals more clutter on a wiki. Others might have been scared off because it gets tagged for deletion.
There's something I've been working on that is really about how we need fan fiction on wikis because it's a creativity that can lead to creativity in other areas. I don't think most people would mind if there was fan fiction as long as it was identified as such.
So one experiment I'd like to try is to specifically allow fan fiction here. Each wiki gets two "namespaces" that they can customize. Namespaces are the prefixes like "Template" in "Template:Delete". We could have one here called "Fanon". It helps keep the fan fiction separate and immediately identifies it as such.
If the adoption is approved, I'll turn in a request for a "Fanon" namespace.