The ARG site "Pottermore" is loaded with Rowling's "ghost ideas" — pieces of the series' world that she fully considers canon but never found a place to be put into the books. Professor McGonagall's surprisingly tragic backstory has gotten one of the biggest fan responses.
Although her initial plans not to kill any of the Power Trio held up, Rowling did consider killing off Ron halfway through the series at one point, when she was "in a very bad place" (although she knew in her "heart of hearts" that she wouldn't do it).
Hermione was planned to have a younger sister who was a Muggle. Rowling eventually decided that it was too late to introduce her, so Book 7 makes it clear that Hermione is an only child.
Among the highlights of what Rowling cut from the series include a dandy named Pyrites working for Voldemort; Sirius in dog form being "adopted" by an eccentric dog-lover (this would have replaced Sirius's plot of having to hide out in a cave), and the character of Mafalda who would have been introduced in The Goblet of Fire.
Fantastic Beasts and Where to Find Them introduced the idea of a "Lethifold" — basically a murderous living blanket that would smother its victim and could only be defeated by using a Patronus (making it an obvious foe for Harry, since the Patronus is one of his best spells). This was revealed during the long wait for the latter books, and many people expected the Lethifold would show up but in the end, it never did.
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Rowling was asked permission to make a Harry Potter musical byMichael Jackson! Sadly, she turned him down. Rowling has always fought to keep Harry Potter as British as possible.
Long before the film's production, Nintendo of all people were interested in developing a video game adaptation of the original book, initially aiming to provide a very British-oriented take in line with J.K. Rowling's wishes. However, the plans for the game fell through, in part due to Executive Meddling from Nintendo of Japan, who pushed for a more Japanese-friendly approach that took considerable liberties with the source material. Ultimately, Electronic Arts would be the ones to adapt the book into video game form, with the initial home console release being on Nintendo's direct rival at the time, the PlayStation (an Updated Re-release would, however, come out on the Nintendo GameCube two years later). After WB had smashing success with creating the Batman: Arkham Series in house, they stopped licensing their IPs for video games. In 2017, they created a subsidiary called Portkey Games dedicated to creating games "inspired by" this universe. The first feature-length game called Hogwarts Legacy released on February 10, 2023.
The movie series could have very well featured an example of Awesome Music by The Boss himself, Bruce Springsteen, called "I'll Stand By You". Springsteen wrote this ballad sometime between 1998 and 2000, being first inspired when he was reading the novels to his youngest son Sam. Sometime in early 2001, Bruce made the song available to director Chris Columbus, who at the time was shooting the first of the Potter movies, and a Springsteen recording of the song was filed with the US Copyright Office on 13 Jun 2001. However, the Springsteen song was ultimately rejected due to Rowling's contractual stipulation that no commercial songs of any type be used in the Harry Potter film series. The song was eventually released and repurposed nearly two decades later for the film Blinded by the Light.
Michael Jackson! Sadly, she turned him down. Rowling has always fought to keep Harry Potter as British as possible.