Tunnel of Foreshadowing

A device by which the audience or a major character (or both) gets a brief view of other major characters who have yet to appear in the story: an optical effect gives the sense of a moving tube or tunnel, down which translucent or opaque images of various characters swirl and recede. When not found as part of the opening credits, the Tunnel is usually the result of some mind-altering experience, such as the process of being Trapped in Another World.

Anime and Manga

 * The opening credits of Prétear.
 * Similarly, the opening credits of Dual! Parallel Trouble Adventure has a brief tunnel-like sequence.
 * Makoto's transfer to El-Hazard in El-Hazard: The Magnificent World.
 * The very first scene in the Read or Die TV series.

Comic Books

 * In comic books, Professor Zoom's time-rips showed Bart Allen in the Kid Flash costume several months before he started wearing it.

Live Action Television

 * In Doctor Who episode "The Parting of the Ways", temporarily super-powered Rose gets a glimpse of the Tenth Doctor (David Tennant) in a similar optical effect, several minutes before Eccleston's Doctor actually regenerates into him.

Video Games

 * Literal example: If you stand in the three tunnels to each island in Crackdown, you can hear the bosses of those islands cursing you out.
 * The openings for Fire Emblem 9 and 10.
 * The Legend of Zelda: Majora's Mask shows silhouettes of the game's various transformation masks flying past the screen after Link falls down a very, very long hole at the beginning of the game.

Western Animation

 * The pilot for Tiny Toon Adventures, "The Looney Beginning", had Buster and Babs open a trunk full of villains, many of whom they would encounter in later episodes.