Some story ideas on the theme of being deceived...
An ancient world of humans believes it is our own Earth in the distant future. In fact it is a colony world -- and its more ancient civilizations, which human history depicts as _human_ civilizations, are in fact _alien_ civilizations, if you go back more than a few centuries. This could result in a big shock for time travelers...
A fantasy novel with two big conceits. First, there was a lost epoch of human civilization during the last Interglacial period. Second, there will be a science-fantasy human future where most humans have died out or moved to other planets. This future will look a lot like a traditional fantasy universe, and at first will be presented to the reader as such. Someone in that time period travels back in time to the primordial period, attempting to influence, however indirectly, the history leading through our time and all the way to his own. Gradually the reader will figure out what's going on. Not really a story yet, just two settings and a mood. Call it _Interglacial?_ Each story takes place in an interglacial period; and of course the intervening interglacial is our own.
"Broken Windows," or "Dread and Circuses." A city in a superhero world is losing its tourist industry because the superheroes (and villains) have migrated from the place. Basically it's been "cleaned up" and the heroes have gone on to other things. Somebody gets the idea of setting himself up as a supervillain to draw in the heroes and save the town. (The first title is taken from the parable of the broken window.)
Cloak of Expertise -- a magic cloak that does not confer any particular aptitude, but grants the bearing and attitude of one of great skill. Both the wearer and the witness believe the user knows exactly what he's talking about. The ironic thing is that this confidence often serves as well as actual knowledge.
The Gratitude Beast. A strange energy creature that feeds on the emotion of gratitude. It grants humans increasingly outrageous gifts, for which it expects equally outrageous outpourings of gratitude. If it does not get what it believes is its due, it will lash out with poltergeist-like rages.
Update: Edited for clarity.