If you can't figure anything else out, assume that the Great Prophecy was a 3-die stunt on various "Of Truths Best Unspoken" rolls, cross-referenced with multiple invocations of "Of Things Desired and Feared" and the various "Auspicious Prospects for <Caste>" Charms.
I'd like to imagine something grander, and all, see. But you could get a Great Prophecy situation pretty simply.
"So, er, what happens to the world?"
(Of Truths Best Unspoken)
"It's heading for lots of suck."
"I get a bunch of Sidereals together." (Description of lots of Big Prophecy foo, with neat prayers and joint rituals and stuff.) "Channeling Conviction, stunting to find sepulchers of future horrors from a long time after the Age of Sorrows."
"Three dice for the stunt."
"Six successes."
"So, that ticker of the end of the WoD running above the White Wolf forums? You see that."
"…"
"How do we avoid that?"
(invokes Of Things Desired and Feared)
"Oh, it's pretty easy. Kill all the Solars, end the time of glory, doom the world to suffering and mediocrity forever."
"…"
"How do we train the Solars to not … do whatever it is they're going to do?"
(invokes Of Things Desired and Feared)
"Sacrifice everything you are; sacrifice everything they are; give yourselves into the hands of fortune; and know that that buys you not victory, but simply the best chance for victory that Charms can buy you."
"…"
"Hey, Battles. Is a war against the Solars auspicious?"
"Seems to be."
"Hey, Secrets. Is it what we ought to be doing?"
"Maybe."
"Maybe?"
"Look, I gotta look wise and mysterious ovah here, kay?"
"Gotcha."
"Okay. Let's upgrade from Big Prophecy to Great Prophecy."
(Description of lots of Great Prophecy foo, which mostly involves setting plans for each of the possibilities into motion and then comparing/contrasting the results of various Auspicious Prospects and Of Truths Best Unspoken results. Result: stunt dice, plus getting to actually look backwards from different futures and do compare/contrast.)
"Yah. There are three paths, and you know how to follow them; but your best odds are really with genocide."
Today's thought on the Great Prophecy (I have others):
Making the future is a craft, not modern art. The Sidereal can't say, "Then Lyta and the Bull of the North link hands and go skipping off into the roses. Suddenly rocks fall and everybody dies."
That's like "I make a sword, only I make it out of lint and it has a mace on either side. Oh, and roses!"
Or "I write a gripping historical romance about an Elvis impersonator out to take over the world with hip thrusts, only he accidentally flies to the moon and has to conquer the moon bunnies instead."
In short, Sidereals can only shape futures that make sense. Which is where free will comes in: it defines what futures make sense, and which ones seem to make sense but then unravel when push comes to shove, and which ones you can say, "Okay, this just doesn't work." as you look at them.
The Great Prophecy might just boil down to "we can build these two futures with a reasonable chance of success, and this third, better future with a tiny chance of success."