What is Media Tie-In Fantasy Fiction?
Until I was asked to write this article, I had never heard the term either. I discovered that it's exactly what I've been writing for decades. The publishing industry term for this type of fiction is "IP" or "Intellectual Property." It's fanfiction with very strict guidelines. And, yes, people can be paid to write it, but by invitation only.
In order to qualify as Media-Tie-In, the author must keep the story to fandom canon. The genre is not limited to fantasy fandoms only, but any fandom of an intellectual property owned by another. However, Captain America won't be spending his years locked in the Arctic actually in Ragnorak. Hobbits won't be cavorting with Ewoks. Kylo Ren and Rey won't be hanging out on earth in high school together. And you won't find 1D, SOS, or BTS in this genre, because those are real people, not intellectual properties.
Many professional authors write Media Tie-In Fantasy Fiction. Some even started there. Delilah Dawson, author of New York Times Bestseller Phasma, admits that her first book she ever queried was intellectual property. She also admits that it was fatally flawed and would never secure an agent. Not because the book was bad writing. Not because it had a Mary Sue. Not because of anything else other than someone else owned the property and she had not been invited to submit the story. (https://twitter.com/DelilahSDawson, 19 October 2017.)
Why even write it then? Well, according to Janine Spendlove, author of the Star Wars story Inbrief, she writes about Hobbits in her free time just to give herself a creative outlet without a deadline. Yes, she keeps the story and the characters to canon. Yes, she will never make a dime from it. But, yes, it's a vital part to her creative process. ("Episode 51: Author Janine Spendlove, Tenacity, and Chasing Dreams." Interview. Unmistakably Star Wars (audio blog), October 4, 2016. Accessed October 20, 2017. http://www.unmistakablystarwars.com.)
In fact, Star Wars is not the only intellectual property to have published authors writing fanfiction. According to the Daily Dot, famous Media-Tie-In authors include R.J. Anderson, Lois McMaster Bujold, Megan Cabot, Cassandra Clare, S. E. Hinton, E. L. James, Neil Gaiman, Naomi Novik, John Scalzi, and Orson Scott Card. (Romano, Aja. "10 famous authors who write fanfiction." The Daily Dot. March 09, 2017. Accessed October 20, 2017. https://www.dailydot.com/parsec/10-famous-authors-fanfiction/.)
You're in honored company. Don't be ashamed. Just do it right. It could be the foundation of a vibrant writing career.