I had an idea for a gig that would entail writing an article for a work of fiction on a Wiki as a promotional tool but I wanted to make sure that was allowed under the current rules first.
You can write anything you wish.
By the way, on the getting off to a good start genre, your profile description contains a URL which is against Fiverr’s TOS. (Terms Of Service) I would remove it A.S.A.P.
Okay, here is the gig I was planning: https://www.fiverr.com/zarpaulus/write-a-tvtropes-wiki-article-for-your-fiction?arrived_from_manage_gigs=true&display_share=true