Hmm. I've got a moment, let me try my hand at this.
Opportune Moment
You have the patience to wait for the perfect moment to strike...
Prerequisites: Base Attack bonus +4 or better, Combat Expertise, Power Attack
Benefit: While taking the full defense action, as a free action you may target one opponent you threaten with a readied melee weapon and roll 1d6. The result of this roll is added to the damage of your next melee attack against this opponent. This damage is aplied after any multipliers for a critical hit. This bonus is cumulative with itself, but may not exceed twice your base attack bonus. Any time you take damage, this bonus is reduced by an equal amount. You lose this bonus after your first attack on this opponent, successful or not, if you target a different opponent with this feat, or if you lose line of sight to your current target.
Sudden Death
Prerequisites: Base Attack bonus +6 or better, Opportune Moment
Benefit: When using the Opportune moment feat, you roll 1d6+1 when targeting an opponent. Further, the maximum bonus you may add to an attack with the Opportune Moment feat is equal to your base attack bonus x3.
Play with Your Food
Prerequisites: Base Attack bonus +8 or better, Opportune Moment
Benefit: Once per attack when attacking an opponent you have targeted with the Opportune Moment feat, you may reduce the additional damage bonus by 10 points to also inflict 2 points of temporary ability damage to the ability of your choice if the attack is successful. This cannot reduce the target's ability score to less than 3, and does not affect creatures immune to critical hits. This damage is recovered at the rate of 1 point per day.
This should taste good for taunting BBEGs, and has some promise for PCs who like style or taking captives

. By rolling the damage when you save it rather than when it's delivered, you can tell how much wammy you've built up. The cap is similar to the existing cap on power attack (2x your BAB with a two handed weapon) so it scales with the user. With successful hits reducing the stored damage, your wizard buddy droping a lightning bolt on him will put a crimp in his style. And finally, the readied weapon and in threatening range insures this feat is acutally used in combat, not studying from behind a wall of minons, or staring at you over dinner before the fight actually breaks out. It IS phrased so you could load up on one opponent, hit someone else a few times along the way (good for swatting annoyances) and still deliver on your target. Because it only affect the very first attack on your target, you have lots of incentive not to screw around on that one hit. Curiously this makes it a good feat for weapon and shield types (who have not been getting the love the way two-handed weapon [3.5 Power Attack] and two-weapon [3.5 Wwo-weapon Fighting] fighters have been...).
Hope this meets your needs, I think I'll go add something along these lines to the next villian book for
Spycraft 
.