Please note that the idea of a prestege class is not to create an entirely new class, it's a 'focus' in the current class. At best, it will possibly change your race, description, a few stats and adjust your abilities.
The idea is that it does not give you any overall advantages, it just gives you advantages here, but handicaps you there.
eg:
Soothsayer: This prestege class derives from either the Psionist or Mage.
Race: other than becoming aged and loosing an eye, no change.
Description: The Soothsayer appears aged beyond years, not that missing an eye helps his complexion. This is a sage who carries a legend of being able to read future events, even those in the immediate future. You wonder if such an oracle could exist, just as you are about to open your mouth to ask him he looks you strait in the eyes with his single eye and nods. (in lesser words, this is an oracle like charicter, it can predict things)
Stats: maybe an adjustment of char/dex vs. int/wis. Or adjust the complexities slightly.
Abilities:
Read: At any time the Soothsayer has the ability to use 'deem' free of mana charge.
Foreordination: During combat, the soothsayer tells the enemy of his demise as he administers it, (reduces enemy complexity/miss factor?).
Prediction(always active): gives a +10% chance to forsee any attack and avoid it completely.
Constant visions:(always active) The soothsayer is constatly bombarded with visions while in combat, this greatly slows his attacks.
Strange eating habits: The Soothsayer is absolutely disgusted by summoned food for no apparent reason (cannot eat summoned food).
bleh, could work.

let's see what you got.