Maybe something like Wild Magic. Strong but with a chance to back fire. For instance, with the Wild Magic buff you can cast any spell for free but each cast has a 20% chance to backfire and do damage based to the caster or an appropriate incorrect target. This could also spawn the creation of a Monster unit with the ability of Unstable Magic Field. Where there is a 50% chance of causing the backfire effect without the benefit of free spell casting. 

This game really could use more spells and Demons and Wizards definitely fills the need:  it introduces two whole new schools of magic and creates a summoning pool that makes being a summoner interesting to play, and gives a regular mage better options as well.  

Another problem with magic currently is the lame "one time" use spells.  Only one hero or sov can have tireless march?!?  Only one can be annointed by fire?  And the vanilla summoning system is awful!

Couldn't there be a trait that makes it possible to cast such spells multiple times?  And if you have the trait you could still increase the second and tertiary casting cost/upkeep of the spell if you were concerned about spell spamming.

The DLC spells could all be unlocked by quests, giving additional material for the game, as well.

Good idea! 

Maybe, but that sounds like poor implementation.  All they had to do was make it non-additive so multiple heroes didn't stack movement bonuses.   Or change the spell to "forced march" and give all units in the army -4 initiative in any battles they fight. 

p.s any modders {  looks up at davrovana} are welcome to use that idea.