A list of the "biggest fandoms" is easy to pull from AO3 work counts. It tells a writer almost nothing. The question isn't where there's more text, it's what a given universe offers someone about to write in it: how many gaps the canon has, how large the cast is, whether the world's rules are legible.
I'll sort popular fandoms into three groups and say, for each, what it gives a ficwriter.
These are worlds that spent decades accumulating internal logic. Harry Potter is seven books, a clear school structure, a house system, and beside it a whole Marauders era the canon only mentions in fragments. Tolkien's The Lord of the Rings rests on thousands of years of invented history: the Silmarillion, the fall of NΓΊmenor, the Second Age between Sauron and the War of the Ring.
What pulls writers here:
The cost of entry is high too: readers of these fandoms notice when you get Sirius's birth year wrong or muddle the geography of Middle-earth. The Tolkien fandom also runs its own hierarchy of sources: The Lord of the Rings and The Hobbit weigh more than drafts from The History of Middle-earth, and the fandom silently factors that in. Before stepping in here, it's worth deciding which layer of canon you stand on.
Naruto, One Piece, Attack on Titan: fandoms with long timelines and enormous casts. Naruto breaks into three periods (the original, Shippuden, Boruto) and supplies a chakra and seals system. Oda's One Piece runs past a thousand chapters and builds a world out of Devil Fruits and the Haki system. Attack on Titan is a closed story with hard logic about the Walls, the Titans, and the inheritance of power.
What they give a writer:
The weak spot of these fandoms is volume. To avoid breaking the One Piece timeline you have to hold a thousand-plus chapters and dozens of arcs in your head. Attack on Titan forgives less: the finale locks all the lines together, so an AU has to be built from a specific branch point, or the canon logic of the Walls and the inheritance of power starts to contradict itself.
If you want to go deeper into one fandom, I broke this down in detail in the guide on how to write Naruto fanfiction.
Fresh fandoms that gathered writers over the last few years: Rebecca Yarros's Fourth Wing (the Basgiath dragon college, the rider-to-dragon bond), Genshin Impact, and Honkai: Star Rail from HoYoverse.
The math here is different:
The downside mirrors that: the lore can shift under your feet, and the fanon you laid down can be broken by the next volume.
Collapse the three groups into one decision table:
There's a fourth axis people often forget β the language of the source. Tolkien and HP have a settled rendering of names and terms, so the reader expects exactly that; anime fandoms still have competing transliterations (Kakashi vs. Kakasi in some languages), and new games often ship with no official localization at all. Before writing, decide which naming system you're holding to, and don't mix two in one text.
The one whose canon you already know best, not the one at the top of the chart. It's easier to write where you can recall, without searching, who stands in what relationship to whom and which rules of the world can't be broken. If you know two equally well, take the one with the smaller canon: seven HP books are more realistic to learn than a thousand chapters of One Piece.
Large popularity means a large audience, and a large number of people who will spot a canon error. In old fandoms a dense consensus has formed: there are things "everyone knows," and a departure from them reads to a reader as an oversight, not a design choice. A younger fandom forgives more, but it has fewer readers. Popularity is about reach, not about the threshold to entry.
Once the universe is chosen and the first scene still won't come, you can hand the description of fandom, genre, and mood to the generator: it removes the blank-page fear and returns a draft that you then edit yourself.
Posts are written by Fanficia's AI editorial team with our author personas.
Describe the idea β the AI drafts a chapter and you stay the editor.
Open the generator βCanon and fanon, the difference: canon is the source, fanon is the community's conventions. On Sherlock and Lord of the Rings examples, with the line where fanon is safe.
A systematized glossary of fandom slang: work status terms (WIP, OS), character terminology (OTP, ship, OC), genre abbreviations, and platform vocabulary.
Core fanfiction genres β romance, angst, fluff, hurt-comfort, crack, gen β how to read tags, how genres combine with ratings, and how many to use in one work.
Andrii ran tabletop campaigns for about ten years; now he tests software and takes other people's universes apart bolt by bolt. He likes it when canon holds together: timelines, magic rules, who's related to whom. He writes fandom guides and explains how to keep worldbuilding consistent even when you're writing past where the authors stopped.