New Feat:

Some people are born with the gift of prophecy. Their poetry (however terrible it may be) seems to gently guide the future in their favor.
Prerequisite: None.
Benefit: Twice per adventure, the prophet can speak a prophecy as a full round action. The prophecy must be in the form of a poem (options include sonnet, limerick, or just sets of rhyming couplets). When the prophecy is relevant to what is happening, the prophet can choose to activate it as a free action which can be taken out of turn and does not require him to be aware of the die roll. This provides a +2 bonus to one d20 roll or damage roll, or a -2 penalty to one d20 roll or damage roll. The prophecy is then expended and cannot be activated again for that adventure. If any prophecies are not expended by the end of the adventure, they were not relevant and are expended.
People with more presence and inner strength can speak stronger prophecies. For every five character levels the prophet has, the bonus increases by +1. For example, a 10th level prophet grants a +4 bonus or -4 penalty. In addition, for every eight levels the prophet can speak one additional prophecy per adventure.

DM Notes:
This feat is “balanced” around an adventure being 3-5 gaming sessions. Adjust frequency accordingly if you run one-shots or sagas with no natural break points.

If NPC’s have this feat (probably just one per adventure) it is good form for the PC’s to hear the prophecy before it applies.

It's tricky to adjudicate whether a prophecy is too vague or if it should apply or not. A feat should be useful, but it's against the spirit of the thing to use
"Something good happened to me
Or something bad didn't, whee!"

Fred the Wizard took the Prophecy feat at first level. Before entering into a goblin warren, he reveals the following prophecies:

Feather light, hold back
Softly touching, surprising
Gentle to wizards

There once was a fighter named Bob
Who made goblin shisk-ka-bob
He stabbed and slew
Till his face turned blue
And money, he made a gob.
The first prophecy is probably going to help reduce damage to Fred. Maybe he falls in a pit but is ‘feather light’ and so takes a little less damage. Maybe a goblin is surprisingly gently while striking him and does a bit less damage. This prophecy would not help Fred read runes, do more damage with a magic missile, or find a hidden door.
The second prophecy will help Bob the Fighter fight goblins. Could be +hit, could be +damage. It wouldn’t help Bob avoid being hit or force open a door.

Commentary: I'm not sure if this is too strong, too weak, or about right. Weapon Focus is a constant +1 to hit, Dodge is a situational +1 to AC but is relevant a lot more often than this feat. This is more flexible than the more typical +numbers feats, but I'm not sure if I limited the uses too much to make it worth taking.