Quoting LikeTheWhirlwind,
I'm personally reviewing all the spell XML and I want to let you guys know that changes are coming. We're looking into separating the hit and damage rolls for offensive spells. The hit roll will be caster INT vs. defender DEF. The damage rolls will be between a min and a max value, not just from 0 - max. Here's the fireball description in my current build:
The only problem I see with this, as others have mentioned, is that magic "missing" seems...stupid, to put it frankly, particularly if it's based on DEF. To cast, I'm investing mana in addition to action points. Granted, I'm doing it without the possibility of retaliation, but using tactical spells still seems like a poor trade-off when there's that much of a possibility of it doing absolutely nothing.
"Target enemy unit is hit with a fireball and is dealt (5-9) damage. Damage is increased by (0.15) for every point of intelligence and by (2) for every captured fire shard."
This. Oh please, God, this would be so amazing. Consistency in magic would be incredible, although the specific formula mentioned here wouldn't appear to scale well at higher levels. I think the multiplication-from-shards can work fine, but the key is to keep magic useful at higher levels. Maybe the level-1 spells won't do you any good, but...well, it all depends.
Also, it's a bit of a problem when sovereigns can be wiped out with one cast from across the tactical map (whoever gets attacked loses 1 essence!) but I think that's more of a problem with the general toughness of sovereigns than spellcasting--you can do the same thing with a few archers.