As a second thought: perhaps spells should also benefit from the "trait" system; so it is possible to assign traits to a spell semi-randomly. "Overkill" would be a rare spell trait, and a spell like Fireball would be guaranteed to have this trait.
- Unresistable (spell trait) - only applies half the applicable resistance score when cast on units
- Death drain (spell trait) - when a spell with this trait deals enough damage to kill a unit, the caster regains the mana expended to cast the spell
Additionally there might be particularly powerful "mastery" traits that a sovereign can gain that allows him to add a trait to all spells he casts. Example:
- Fire mastery (trait) - any fire spells cast get the "burn" (spell trait) ability that deals additional damage a round later based on spell level
- Zenith of life (trait) - all friendly unit enchantment spells gain "regenerative echo", this grants enchanted units faster healing
Ultimately this could lead to the ability to have a spell designer, similar to how you can create your own units.