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Writing within an established universe may be fine, especially for drafting. If you start out with some borrowed characters or concepts, that makes it easier to do the parts YOU want to do uniquel...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/48685 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/48685 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
_Writing_ within an established universe may be fine, especially for drafting. If you start out with some borrowed characters or concepts, that makes it easier to do the parts YOU want to do uniquely, and then as you write it may become your own. It's like having the 5-paragraph essay structure -- not great for in-depth collegiate papers, but a great starting point, and a good way to cope with essay exams. Some writers DO focus on "tie-in" and "licensed" works within a franchise. Often at SF cons there may be a panel on the pros and cons of working with that structure and maybe how to get started. (Keith R.A. DiCandido often is on these panels -- I met him at Balticon before I knew he wrote the Trek "Core of Engineers" sub-series -- and here's his mostly-updated [bibliography](https://decandido.wordpress.com/2019/01/30/the-compleat-bibliography-of-keith-r-a-decandido-4/), organized by world/franchise.) I don't know the path to become one of these "official" in-universe writers (apparently it must involve not sleeping, as he does cons most weekends AND several weekly columns AND all these books and other published works, and he's polite and active on social media), but I have bought some writer's independent works because I like their franchise works, and vice-versa. Harry Potter would be tricky, as JKR is still producing works in that universe (although she had a collaborator for the play) and has been known to issue Cease & Desist letters to fansites focused only on HP. (Fanfiction.net and ArchiveOfOurOwn seem to be safe.) I proposed a book to a publisher I know about "Lessons from Harry Potter" (for educators to write about things one can learn from the series - so strictly analytical essays, no in-world writings), and he refused it stating concerns about JKR's litigiousness. U.S. Copyright law is typically lifetime+of+Author + 70 years for all works after 1978: other guidelines applied from 1920s-1970s (often involving renewals which frequently lapsed), but typically, in the U.S., anything pre-1924 is fair game. (Original Oz books!) Very modern works (21st century) may have a CopyLeft, Creative Commons License with Remix/Derivative works options, etc. [Cory Doctorow](https://www.craphound.com) frequently has released his works on CC and believes that it's essential to all creation: [https://craphound.com/news/2016/12/08/everything-is-a-remix-including-star-wars-and-thats-how-i-became-a-writer/](https://craphound.com/news/2016/12/08/everything-is-a-remix-including-star-wars-and-thats-how-i-became-a-writer/) (a link to a podcast where he discusses those topics.) Doctorow's website used to more prominently display "Pay What You Want" and "Share What You Made" and other CC options. I just wanted to provide an alternative to simply "No, don't do it."