Most of my fanfic has been about episodic TV shows, that have limited continuity from episode to episode. There has thus been plenty of opportunity to insert big adventures into the gaps between episodes. I've written loads of long stories, but even my shorter stories have usually had some sort of plot, even if the plot is a fairly basic one designed to support character exploration. What I don't think I've ever written is an entirely plot-free bit of character study or character interaction - a "this is how character X was feeling about the events in episode Y." Such stories have been entirely supported by the source material; I've just not been felt like writing them.
I've always had some qualms about writing novel-based fanfic. Partly it's because playing with a single author's creation feels different to me from playing with something shaped by a group of writers and actors. Mostly, though, it's because most of the books and series that I love tell a closed story that ends with an end, and I can't find any gaps I want to play with. I wrote Dark is Rising fanfic, but that was all "20 years after" sort of stuff, dealing with the consequences of the ending. I also wrote Diana Wynne Jones fanfic, but the Howl and the Chrestomanci stories don't tell a single coherent story, but are pretty much self-contained novels in which Howl or Chrestomanci pop up half way through in a strong supporting role, so they were completely ideal for an outsider viewpoint fanfic.
I always thought The Lord of the Rings offered me no gaps. While I could quite happily write an epic adventure for TV series characters, taking part between 2 named episodes, and I will not write an epic adventure for the Fellowship taking place between two named chapters. Thinking about it further, I find that they are gaps into which adventures can be fitted, of course - Aragorn's backstory, for one, or the entire Fourth Age. But, still, opportunities to write my usual type of fanfic are limited. I can't see myself writing those historical or post-apocalyptic AUs that I'm so fond of. Added to that, I'm far less comfortable with h/c in this fandom than I normally am - not opposed to it, but I need it to be a lot more understated than normal.
And of course there are loads of character-based gaps. Tolkien's story can be very moving, but he devotes a lot less time to his characters' emotions than fanfic likes to do. There are therefore loads of opportunities to delve a little deeper into thoughts and feelings, or to do precisely those "what was character X thinking after episode Y" sort of stories that I've never done before. There are minor characters to explore, and loads of hinted-at backstories. In a way, I'm outside my comfort zone, but I think it could be quite fun.
In other fandoms, my Muse has spent most of her time mulling over whatever my next Long Story will be. With this fandom, she's currently a little overwhelemed with an amorphous stew of half-formed ideas, I think she'll need a lot of prodding by challenges and contests and the like. I've written an entry for the next Teitho challenge, but that's all Sekrit so I can't talk about that. And I've written a story for this month's lotr_community challenge, which was the first time I've ever written a story in response to a picture, and also marked a very rare attempt to write to a very low word limit, in this case 1000. Here it is. (1000 words featuring Bilbo and Aragorn in Rivendell some years before the start of The Lord of the Rings.)