My fanfics:

Mouthwashing (One-shot)

What if Curly and Jimmy swapped narrative roles?

Punchline (Batman)

Batman and Joker meet again to develop their dynamic once more, when will this end?

Other original texts that are not fanfiction:

Chronicle

Star Trek non original work special section, an homage to the start of modern fanfiction:

A Fragment Out of Time by Diare Marchant (1974) [NSFW]

First slash fan fiction, since slash ships started with the ST fandom, with Kirk/Spock (or K/S, also known as "The Premise") specifically. Published in the zine "Grup" #3.

A TREKKIE'S TALE by Paula Smith (1974)

"Mary Sue" archetype's name origin.

The first Star Trek's fan stories, printed in fan zines specifically, were the ones from Spockanalia's issue #1, from 1967.

World's first known "fan fiction" writer, Publius Vergilius Maro (Virgil). His "Aeneid" is an Homer's "Odyssey" fanfic. Hey, I also shout-out to Dante Alighieri for "The Divine Comedy"!