Well, in the vein of the metabreath feats in Draconomicon, I'd suggest the following:
BREATH SUBSTITUTION [METABREATH]
You can modify an energy based breath weapon to use another type of energy instead.
Prerequisites: Con 15, breath weapon, any other metabreath feat
Benefit: Using this feat, you may chose to have your breath weapon inflict a different type of energy (acid, cold, electricity or fire) than it normally does. All other aspects of your breath (size, shape, amount of damage) remain the same.
When you use this feat, add +2 to the number of rounds you must wait before using your breath weapon again.
Special: You cannot apply this feat to a breath weapon that does not use one of the energy types listed above.