So, let's say...
SelfProclaimedProphet says to LittleGirl, "Last night I had this mjorly bad vision. In it, there was a blue-eyed egyptian man who had hands of fire. One second he was just sitting there real calm, the next he was walking through the NLM building and everything he touched burst into flames."
LittleGirl says, "Whoa." and remembers this story. Maybe writes it down or tells someone.
Then somebody new comes through chargen, looking to have a fresh start with a bombastic character. Things could happen two ways:
a) They make a character who, out of their random selection, happens to be a blue-eyed egyptian who likes pyrotechnics.
b) They ask the GMs for suggestions (or another player?) and are somehow led to be aware of this prophecy/story thing. They build a character to fit the description.
Either way, the character emerges from chargen and is in some way introduced to the fact that this prophecy exists, and that someone out there thinks it's about him.
It's then up to the player to decide wether to make good on the prophecy or toy with it a while and then disappoint the people who thought they were 'the one'.
The pluses of this would be people could walk into plots (GM or character generated) having to do with these prophecies. Gives them guidance and something to play off of if they need it. Gets those who have heard the prophecies involved too.
The minuses are that people shouldn't need extra prompting to make stories and agendas for their characters. Also that somehow getting the prophecies written down or a working grapevine could be tricky, lots of coordination involved and I know many folks are hard pressed to find the time for that ICly.