Stream of consciousness here, several ideas that I've been mulling over for a while.
Sky predators that resemble box jellyfish that feed on birds. Eventually, they get large enough to take down planes. Their previous life cycle stages would likely have some kind of weirdness, I haven't thought of something yet. Likely something involving lighter than air gasses.
An object that, when held, makes you impervious to damage. However, as soon as you let go of it, all damage you would of sustained immediately occurs, with the appropriate exhibition of energy. IE, bursting into flames after being lit on fire, flying back after being shot with a powerful weapon, flying into pieces from an explosion, ect.
A formation of ghost aircraft that under no logical circumstances should be flying, all being from recorded crashes. The victims are still alive, but as soon as they are taken out of the formation, they die from their wounds, or, if they are still in surviving condition, the bends.
A device that converts energy from one form to another with 100% efficiency, depending on the setting its knobs are on. It absorbs all energy that it is set to, and emits the type of energy that the other setting is put to.
A steel humanoid figure with a music box in its head. It is incapable of movement, but plays music at irregular intervals. Upon heavy analysis of the music, it is discovered to be a code/language, with bizarre prophecies being stated. I'd actually make the sound file for this.