How about letting mages (or perhaps a new guild for this purpose) create their
own spells. Basically for offensive spells we would need a bunch of manatypes,
power of spell, time to cast etc. Features that mage him/herself could choose
for his spell, and based on those the casting cost would be calculated.

This same idea could be applied on protective spells too. Perhaps with certain
limits or cumulative casting cost, so that no overkill spells could be
created...

Some skill would be needed to alter the construct of your spells, and a mage
would be able to have only a limited amount of "own" spells at a time. To
create a new spell he then would have to discard one he has created.

I believe this would turn out very interesting, especially if the amount of
properties a spell may have would be increased, here's a couple suggestions:
spell that steals life/mana from enemy and distributes it to party/caster.
Piercing, which would lower the enemy's resist to that spell by 20% or so.
Spell that does 2-3 types of damage insted of one, Spell that reduces enemy's
attacks for couple rounds and so on...

The basis of whole system would just be an efficient spell cost calculator.
The more properties, greater dam and shorter the spell, the more expensive it
would be. Thus very powerful blasts with long cast time would also become
possible.

-Tsuran