I'd love to think of a solid ruleset for prophecies occurring in-game. For a long time I've been fiddling with various ideas (a huge d1000 table of random events which you roll and consult when a prophecy is uttered - the selected event then has a 5% chance of happening whenever a random encounter dice is rolled; or a random generator which would come up with results resembling the kind of thing fortune tellers might talk about - "a tall dark stranger", "a white cat on a table", "an old man with a dog", whatever - which you would roll and consult, and then if the players encountered such a thing in-game they would receive some bonus or penalty) but none of them seem quite right.
Recently it occurred to me that it might be best to simply write up a random generator which would generate "tall dark stranger" or "white cat on a table" type results, and leave it up to the DM and players how to interpret it. Once the prophecy is uttered, you know it will come true, and the DM has it as a kind of ace in the hole that he can bring into the game whenever he feels like it. But the meaning and effects are dependent on the players' reactions, the context, and what the DM thinks would be interesting.
Thus, a set of oracle results which I just threw together:
The idea being that you roll 3d10 and consult. The first two results provide an image ("you see a figure swathed in blood"; "I see a tower gleaming with light", etc.) and the third a feeling associated with it ("it fills you with a strange sense of peace"; "I feel a deep feeling of regret emanating from it", etc.), and the DM is free to embellish as he sees fit ("You see a figure swathed in blood. You can't see its face, because it is covering it with its hands. But its garments are soaked through with crimson. You have the distinct impression, somehow, that it hates you.")
Needs work to provide more bases and modifiers. The feeling column should likely be shortened or perhaps done away with altogether, so it can be left for the players to interpret. But the principle is there; the challenge is, of course, for the DM to work out when to bring it into the game - but that's the beauty of prophecy; you don't need to specify a time. The Mayans should have spotted that, really.