On the subject of famous people writing fanfic:
10 famous authors who write fanfiction,
12 successful SF authors who’ve written racy fanfic6 famous authors who’ve written fanfic, and hilariously,
4 famous authors who wrote fanfiction. That last one being people in history, rather than anyone terribly current.
I’ve talked before about how singers do this too, write fanfiction to sing, but it seems a little more acceptable or mainstream for them to do it. More people expect singers to sing about what they see or experience and more authors are expected to use imagination.
Sting: Moon Over Bourbon Street; inspired by Anne Rice’s Interview With a Vampire
The video of Madonna’s Material Girl music video was a mimicry of Marilyn Monroe's performance of the song "Diamonds Are a Girl's Best Friend" from the 1953 film Gentlemen Prefer Blondes.
I’m sure I’m missing great amounts of examples, but there you go. If anyone knows more, feel free to list them in the comments.