Just in case you're actually unaware, the Organization for Transformative Works https://archiveofourown.org/ Archive Of Our Own (typically shortened to AO3) is where a tremendous amount of such fiction is archived.

This is not an endorsement of the work, but there's Harry Potter and the Methods of Rationality. I hear 50 Shades of Gray is another fanfic that went mainstream.

A book nerd could come up with a much longer list, but I know there's a ton more illegal unlicensed! Harry Potter fan fic.

exactly.