Beyond Selidor

Not a problem at all. Generally there's not an 'expectation' for others to write free fanfic — most people who write it are writing it for a combination of reasons (love of the characters or canonical material, the joy of writing, the sense of community with other fanfic writers and readers). The only instance in which there is an expectation or obligation is for fanfic exchanges, where someone (or a group of people) organise an event in which participants write a specified fanfic (e.g. a minimum of 1000 words, or 5000 words, or whatever) based on being matched to other participants' requests. Prior to this, everyone who wants to participate nominates canons and characters that they'd like to read and/or write about, every potential participant sees what has been nominated, and then makes offers and requests based on that tagset of nominated fandoms and characters. So what results is an exchange of gifts — or, as you say, a trade of work.

My post above was about one such gift exchange, waybackexchange, in which participants request and write for fandoms which have had no new installments for at least ten years (e.g. old books, movies, TV series and games). Other gift exchanges might be just for fanfic about Marvel film characters, or female characters, or for stories where worldbuilding is the main element, for example. But the vast majority of fanfic is not written as a gift to someone else — it's just written by people when their creativity is sparked, and if it finds an audience, that's just an added bonus. Probably what you're seeing in other people's journals are their 'Dear Author' letters for fanfic exchanges — where they can flesh out their requests beyond just their requested characters to include more details about the kinds of stories, tropes and themes they enjoy.

I hope that makes sense. There's a lot about fanfic community norms that is probably weird or inexplicable to those outside the community.