I suggest that something like the following be added to most of the upcoming spellcaster specialty prestige enhancements:
Fire Savant I
In addition to the other effects, whenever you inflict fire damage on an enemy, it suffers +20% damage from fire for the next 6 seconds. A successful Will save reduces the increase by half.
Fire Savant II
The 20% increase to fire damage you apply to enemies can stack with itself once, for a total of +40% while both are active.
Fire Savant III
The 20% increase to fire damage you apply to enemies can stack with itself twice, for a total of +60% while three are active.
Obviously all the other sorc specialties would have an equivalent, and some wizard specialties might share that idea in a weaker, generic way. And I'd also say that the Sacred Exorcist Cleric enhancement could provide that effect, applying to alignment damage spells (Good, Law, etc).
Gameplay Results
This effect gives a good increase to the spellcasting DPS of those characters, but doesn't give them any help in wiping out weak mobs with a single AOE. It only applies to fairly strong enemies, who already last long enough that it takes more than one spell. That is the kind of opponent for which spellcaster DPS most needs help.
Also notice that this change is a party-friendly effect, which improves DPS not just for the character who has it, but also for other spellcasters, and even weapon attackers using items with corresponding energy damage.