Shorts can take many forms: cartoon, parable, excerpt, vignette. Sometimes the teaser for a film is the film itself. Trailers are created for nonexistent movies with no intention of expanding it into a feature-length production. Popular examples are the World's Finest trailer for a Superman/Batman team-up, as well as Grayson, a project by the same team that puts Robin in the leading role after Batman's death.
It's easier to create a standalone trailer based on an existing property, as with so little time in which to draw viewers in, using familiar characters will quickly bring them up to speed sufficiently to appreciate the tale being teased. At the same time, creative types can still use such trailers to reinvent established franchises, taking them in bold new directions. No more dramatic effort has ever been witnessed than in this trailer for Office 2010: The Movie.
Even if the popular yet despised productivity suite is not destined for the silver screen, it does have its share of stories. At ROFLCon II, I had the pleasure of attending a presentation by Kevan Atteberry, creator of Clippy, the much-maligned assistant found in Microsoft Office 1997–2003. He disavowed responsibility for the loathing Clippy incited — "I only invented the chracter; I did not invent the functionality" — but is nonetheless happy with the fame it brought him, saying that he still gets 3-4 letters a year about Clippy. "It doesn't matter to me if you like him or not. As long as you know who he is, I'm happy."
Should the above trailer ever prove fodder for a feature film, expect fans to be dismayed at the source material being betrayed: