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First thing you would need to decide is what you actually want to do with that sequel. Do you intend to write something that is basically "more of the same"? "More of the same", but more modern? Or...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/42363 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
First thing you would need to decide is what you actually want to do with that sequel. Do you intend to write something that is basically "more of the same"? "More of the same", but more modern? Or do you want to deconstruct the source material in some way? All are viable options. In either case, there are a few things you want to be very careful with: existing characters, and themes. **Characters:** you don't want characters from the source material to suddenly start acting and thinking very differently from the way they did in the original, without good reason. You want them to be the same familiar characters, not different characters that just happen to carry the same name. As an example, _Star Wars the Last Jedi_ received a lot of criticism because Luke is so different from the person we knew him to be in the originals. (The film tried to offer justification for his changed attitude, it's up to you to decide whether this justification was sufficient, and whether the change was a good idea in the first place.) **Themes:** if your work is very different in spirit from the source material, fans who loved the original for what it is might respond negatively to the significantly different sequel. This is another reason _The Last Jedi_ was attacked - it stepped away from the straightforward idealistic black-and-white hero's journey the original trilogy followed. (Again, not getting into whether it did so well or not.) Whatever you write, if the original work has a significant fandom, there would be those who would feel your new material is "sacrilege", and nothing can compare to the original. There's nothing you can do about those voices, other than accept them as the price of writing a sequel to another's work.