I think fanfiction as a medium is different enough from mainstream literature in the tools it offers writers that it's a shame that it's not talked about more often. And it's not me saying "fanfic is better than books xD" because that sort of mindset is a symptom of people who aren't particularly well read in either medium. I'm just speaking of like... The little things you get to do with a fanfic that you genuinely can't really do in an original story.
I had a big fanfic in a previous fandom where one of the big reveals was the involvement of a kind of infamous villain, whose presence was built up to and foreshadowed through the whole fic until his reveal without ever mentioning his name, so that the name drop would be a gut punch. It worked especially well because of who the villain was and his presence in that fandom space specifically (it's very complicated) and if it was an original story this reveal wouldn't work at all the way it was written in the fic. Because if you don't have a predisposition to think about that character and his relationship to the hero in a very specific way, then just seeing their name won't do much to you; the reveal and the recontextualisation it pushes upon you hinges on your previous knowledge of the source material.
I think it's an interesting tool fanfic authors are given. One of my favorite fanfic of all time is partially a re-imagining of its source material's canon, and something it does is introduce antagonists much earlier in the story or deepen npcs' stories. It then works to evoke a tragic irony that again wouldn't work if you didn't know the source material, and it's something the author obviously has a lot of fun with.
You could call it cheap or a crutch and I mean, yeah, sure, it is a little bit: the fanfic relies on previously established emotional bonds and stakes to achieve its goal, and in some cases it saves the author from having to 'properly' build up its stakes. But I think it's INTERESTING that it has that tool at its disposal. I think it's a fun thing to play with and I think these built in expectations and emotional bonds are especially why I find story driven aus in particular to be fascinating in the amount of ways you can play with them. You know??
THIS!!! One of my favorite things to think about when writing is how you create tension to drive the narrative forward. Of course this can be done in a number of ways with interpersonal dynamics or the "surface plot" as it were, but where fanfiction really shines in this regard is DRAMATIC IRONY! Like in Romeo in Juliet where *we* as the audience know Juliet is only sleeping but Romeo thinks she's dead. As a fic writer you can operate under the assumption that your audience has a certain familiarity with the source material and you can absolutely play that to your advantage in ways you never could in original fiction.
OP's example of having a big bad we all know haunt the narrative until their reveal is a classic. One of my other faves is (especially good in an AU) recontextualizing events or dialogue from canon in unexpected ways to either give catharsis or to create an ironic plot twist.
I suppose I've seen it done where it can read as a storytelling "crutch" but in the really solid fics I've read, my all-time faves, the author does this so elegantly. In some ways I dont consider it THAT different from when a lot of 20th century and earlier authors assume their readership came to the table with a knowledge of the classics & so could be assured they would "get the reference" if they drew parallels to the Odyssey or something. Or when authors have little references or intertextuality that shows their piece is clearly in dialogue with their contemporaries. Anyway I love fanfiction as viewed as its own genre of fiction.
