Here's what we have currently.
Cold: A power of this energy type deals +1 point of damage per die. If the power allowed a Reflex save it is changed to a Fortitude save
Electricity: A power of this energy type provides a +2 bonus on manifester level checks for the purpose of overcoming power resistance. If the power requires an attack roll you get a +3 bonus if the target is wearing metal armor. If the power allows a save the DC is increased by 2.
Fire: A power of this energy type deals +1 point of damage per die.
Sonic: A power of this energy type deals -1 point of damage per die and ignores an objectís hardness.
With this system there are "minor elements" (equivalent to the existing ones) and "major elements" which are more powerful. A character picks one major element at 1st level - this is the only element he can apply to psionic powers.
Extra Element [General]
Benefit: Select one major element or two minor elements. When you manifest a power with an elemental descriptor you may use any of the elements with which you are familiar instead of the one you chose at character creation. If you select a minor element which you already possess then it is upgraded to a major element.
Normal: When manifesting powers with an elemental descriptor you may only use the element which you chose at 1st level.
Special: You may select this feat multiple times. Each time you may choose new elements or upgrade minor elements to major ones.
Double Element [Metapsionic]
Prerequisite: 2 elements
Benefit: You can expend your psionic focus while manifesting a power to apply two elements at once. These can be minor or major elements. The power gains the benefits and drawbacks of both elements at once, and gains both descriptors. The saving throw DC of such a power is also increased by 1 (if it does not allow a save, you instead gain a +1 bonus to the attack roll).
If you don't want to use this system wholesale, here's a feat for you:
Advanced Element [Psionic]
Benefit: Select one major element from the list below. Whenever you manifest a psionic power which allows a choice of energy type, apply the effects of the chosen element instead (you lose the ability to apply standard energy types to such powers). In addition, the pp cost of powers affected by this feat is reduced by 1 (minimum 1, this does not affect the number of pp you can spend on a power).