In any story building off pre-established events, whether that's fanfiction, alternate history, speculative future fiction, twisted fairytales, or some other re-telling, there has to be a point of divergance.
At some point, something happened or changed to create the events of the story, and it's important to establish what that was, in order to determine the next important thing: what happened as a result of the change.
I explore this at length in my article about Jane Austen Fanfiction, but upon reflection, that was rather limiting, and I decided to do a more in-depth version.
What Is A Point Of Divergence?
In any story based on pre-established events, whether fictional or real life, there needs to be a point of divergence, something that happened differently to the original timeline.
It can be anything; a missed connection, or overheard conversation. Take History, add dragons (or other supernatural/mythical beings). A death thwarted or postponed. A discovery that was never made, or was made, but earlier or later in the timeline. A natural disaster of some kind, or a man-made one that did or didn't happen, contrary to history. Historical events whose outcome was changed either subtly or dramatically.
The limit is your imagination.
The Ripple Effect
The thing about changes, however, is that they never stop at a single change. Every change causes other changes.
If World War One failed to occur, for instance, Civil Rights gains would be pushed back significantly, because there wouldn't be as much need for women in the workforce, White soldiers wouldn't have fought and died next to Soldiers of Color and wanted better for their brothers in arms. Innovation would also likely be delayed, because aside from the arms race, War leads to a lot of inventions to compensate for a smaller available workforce.
If Ned Kelly's planned uprising had succeeded, or even gained more support, Australian History might have changed radically. Perhaps Australia gains Independance earlier, as Great Britain decides it's too much trouble to put down a rebellion on the other side of the world. Perhaps Ned Kelly makes himself King of Victoria (settlement was still rather sparse at the time, and communication was slow), or perhaps we get Independant East Victoria and Colonial West Victoria, similar to Germany after WWII.
If Alexander the Great had an heir before he died, or lived long enough to see his son born, the Macedonian empire might never have been split up, which may have delayed or even prevented the rise of Rome, if the dynasty lasted long enough.
If there's a second Great Flood (or other Natural Disaster) that wipes out a fair chunk of Europe, North America, assorted Island nations, and most Coastal cities, what is left? Which countries have just lost major industries? Which of the ones remaining are in the best position to take advantage of the upheaval and become New World Powers? Which Old World Powers devolve into Anarchy, and which ones manage to claw back a sense of stability?
If the Flood (or other Disaster) affected the landmass of the entire world, who managed to survive? How? How are they affected by being lone survivors? Are there enough survivors that they band together out of desperation, or so many that conflicts arise over sparse resources? Who gives up? Who rises to the top?
In fandom terms, if Harry Potter was raised by someone other than the Dursleys, away from the Wizarding World, he's likely to be better-adjusted, and potentially have a wider circle of friends than just Ron and Hermione. He's also likely to have more trust in Authority, having not been let down by it so many times, and be less likely to take responsibility for fixing everything upon himself, knowing that he can rely on other people. That Harry may find the Hogwarts professors failing to take him seriously in Book 1 more of a shock than the resignation that Canon!Harry did, which presents a number of possibilities.
Alternatively, a 10th Walker in Lord of the Rings who saves Boromir from dying probably won't have that much effect on the Fellowship breaking. Boromir was already planning to travel back to Gondor - as one of it's generals, he's needed there more than on a stealth mission - and if he can't bring back the ring, a Healer who can bring people back from the brink of death is a damn good alternative! With that in mind, a girl who randomly appears out of nowhere and knows far more than she should, is more likely to be locked up as a potential (if clumsy) spy than allowed on a Top Secret Quest to Save The World.
If Anakin Skywalker never falls to the Dark Side when Palpatine reveals himself, that doesn't necessarily mean that the Jedi live and everything is fine. The Clone Army still has chips and trigger commands, the majority of the Senate and the Galaxy at large are already looking at the Jedi as the source of their problems, and all of Palpatine's mechanisms don't vanish just because he's dead. Anakin still struggles with his temper and impulse control, and he and Padme argue more than they don't. Just because he didn't Fall then doesn't mean that he never will... The Rebellion was in motion before the fall of the Jedi, though, and maybe Obi-Wan and Yoda aren't the last Jedi around, so some things change, but others stay the same.
TL;DR - Every change you make is going to trigger a bunch of other changes, and you need to think about how those changes are going to affect the plot.
This is not to say that you shouldn't write that fanfiction, or alternate history, or Spec-Fic.
But you do need to think about what you're writing, and what is going to happen as a result of the changes you write. Flow-Charts are very helpful in this respect. You don't have to completely plot it out - Fate knows I've had enough instances of suddenly realizing that if A and B take place, I suddenly need to work out what impact C is going to have - but you should at least have a vague idea of the main points.
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