What's the difference between canon divergent, alternate universe and mirror-verse? Sorry if this is a dumb question, but I'm confused because they all seem like the same thing to me.
Not a dumb question, though I’m going to answer this from my own perspective in the Mass Effect fandom so other people might disagree with me or not like my examples.
Alternate UniverseAlternate Universe takes our characters and places them into a whole new world. It’s essentially original fiction with the fandom characters thrown in with new lives, jobs and worlds. The world should be unfamiliar, but the characters familiar.
For example - an AU fanfic about Shepard and the crew might be about them running a modern day sushi restaurant in London. Shepard is the boss, Kaidan and Garrus are chefs, Traynor and Tali are service girls and James is the customer who always eats too much and flirts with the counter girls.
The story might be about them going into competition against the large sushi chain Cerberus who set up shop just up the street! Run by the Illusive Boss, their store is managed by Kai Leng - an unscrupulous man who will stop at nothing to crush the competition.
Mirror-verseA mirror-verse fanfic will take place in a universe that mirrors the world of canon in terms of locations, characters and situations, but where some big shift in canon history has occurred. The world and characters should be familiar, but slightly warped.
For example a mirror-verse fic could be a situation where Kaidan replaces Jack as Subject Zero (like in my fic!) and it’s him they pick up off the prison ship in ME2. Or it could be a world where the Alliance is evil, the crew of the Normandy are evil, and Cerberus are freedom fighters. Or it could be a world where the humans won the first contact war and the Alliance now controls the council along with the Asari etc etc.
Canon DivergentCanon Divergent stories will follow the fandom canon and then deviate at some point, which creates a completely new timeline where anything could happen - though canon events will still occur but with different outcomes. Yes, things will be familiar and a large part of the story is canon, but it is set in the same world with the same characters - but one choice or action is different to canon and this changes the story. The world and characters are familiar, but the story line becomes unfamiliar.
Eg. Commander Shepard never dies above Alchera and fights the Collectors with Alliance support. They know the reapers are coming and the Council believe her and prepare. They find the crucible plans (and other weapons) far earlier and are ready for the reapers when they arrive.
Hopefully this helps! Feel free to add to this or shoot me more questions!