Why would we need a fediverse alternative 😂 unless you want to go around editing wikis with your mastodon account, wikis are basically just static webpages. It's called HTML 😂
Wikis like Fandom typically make it easy/easier to create and edit pages. Wiki editors are rare and you want to make it as easy for them to latch onto helping and volunteering as is possible.
Maybe a fediverse option isn't needed (though the below comment's point about having a central account would make it easier for a lot of users), but having a convenient and easy way to create a wiki for your favorite fandom, without using Fandom would go a long way toward breaking Fandom's hold over the entertainment sections of the internet.
Believe it or not, Wikipedia actually does use a single global account for all of its wikis. That includes Wikipedia, Wiktionary, Wikimedia Commons, and almost every other site listed on their website. The only one you have to create a separate account for is their test wiki where they test new versions of MediaWiki (the software they develop for their wikis).
Jimmy Wales actually isn't as directly involved in Wikipedia as he once was. These days, he only holds the chair emeritus and the founder's seat on the Wikimedia Foundation Board of Trustees.
His chair position changed to a chair emeritus in 2006, and the site implemented the unified login feature in May 2008, though they'd been discussing implementing the feature since 2005.
I learned about this plug in the other day. Indie Wiki Buddy. It's not a fediverse alternative, but it's the next best thing. It can redirect your Fandom results to their independent alternatives.