Side Note, Full Story

When a notion mentioned in passing in one work becomes the full story of another work by the same author. Most often a book trope.

Comic Books

 * Nevada - Courtesy The Other Wiki : "The origin of the character is to be found in a Howard the Duck story that contained a "mandatory fight scene" between a Las Vegas chorus girl, an ostrich and a standing lamp. Neil Gaiman said he'd like to see that story. So when Gerber was asked to come with something original by Vertigo editor Karen Berger, he created Nevada."

Literature

 * Terry Pratchett - The Amazing Maurice and his Educated Rodents (started out as a passing reference in Reaper Man)
 * The Star Wars expanded universe does this kind of all the time. Not only is a backstory provided for every character who appeared onscreen in the movies (and even some of the Faceless Goons), but you also get stories that explain exactly what a "nerf herder" is.
 * China Mieville's Bas-Lag Cycle is full of these. Among a great many other things, the protagonist of The Scar is mentioned in one line of Perdido Street Station as being Isaac's last girlfriend.
 * Sauron was briefly alluded to in The Hobbit.

Video Games

 * Douglas Adams - Starship Titanic (started out as a passing reference in one of the The Hitchhikers Guide to The Galaxy books)
 * The Legend of Zelda: Ocarina of Time is an expansion of the prologue detailed in A Link to the Past's intro cutscene. There is one deviation from the prologue, though: in Ocarina of Time,.