I've always wanted to try using a "20/20 Hindsight Prophecy"...
In which there is a prophecy, and perhaps the PCs know that there is a prophecy, but they don't necessarily know what the prophecy says. I'd let the PCs play through a large chunk of the campaign, and then reveal the prophecy to them... A prophecy that I'd have only just then created, and reverse engineered to match their exploits.
That's how I did it in my campaign... for about the first 20% of the campaign, the players didn't even know about a prophecy. Then, they met some fanatical bad guy who went out with a smile on his face saying "the victory of light shall foretell their doom" or something like that.
Then, maybe 1/3 of the way through the campaign, I revealed to them that they were indeed heroes and heroines of prophecy, as the prophecy had (amazingly) seemed to have been written about them, though I kept it kind of vague and left wiggle room in there for potential PC death. The person who revealed that to them only knew some of the prophecy, as she had been the one the PCs had to save to be revealed as said heroes. Even then, I had wiggle room, as the prophecy said, "bravely defend the heart of the wood" - the woman could have died while they were defending her and they would have technically fulfilled the prophecy, as they did defend her and she was in the center (heart) of the forest. Heart of the Wood could have been a person, a location or a magic item, however.
From there, they later found more bits of prophecy - this time, speaking of "three miracles" that the heroes would perform that would be a sign for former enemies to unite against a common foe and allow the heroes to defeat the wearer of the mighty evil artifact, The Dark Crown, before the BBEG could invoke its full power and enslave the world. I kept it pretty vague over all, however. While I had a general idea of what I wanted the three miracles to be in game, it was not set in stone and was very willing to go with the flow if the players did something special.
Example: I had a line in there about the Blessing of the Hawk shall provide a saving grace in darkest times (or some crap like that). I had intended it to be help via Resurrection from a priest whose symbol was a Hawk. However, it later became a new PC that was added to the group when one player's fiancee came aboard, and she created a PC that was a follower of that same deity (totally unbidden, as the group did not know about that bit of the prophecy yet)
However, the actual prophecy was generally revealed over time, and I also made it clear that the bad guys had an evil version of the same prophecy that was told from their perspective, so they might not be going by the same words.