[jeffr]

If some unit/spell (in this case Wrathful Skies) is used by everyone, then it must be more powerful in relation to other units/spells. In this case, it's value for the cost(i.e. bang for the buck) should be reduced. You could decrease the bang, up the bucks (casting cost, casting level, research level, etc.) or both.

Units/Spells that are not used by anyone or seldom used should have their bang for the buck increased. Increase the bang (unit/spell power) and/or decrease the cost.

I believe Zen's mod attempts to do this.

Maybe in Dom3 there would be some way of capturing MP statistics on what spells/units were used the most/least. Over time, this would give a pretty good indication of what needed to be scaled up and what needed to be scaled down.