One mechanic I really like in MoM was that some of your trained units (e.g. Priests, Mages) had a spell that they could cast once in a battle, at no mana cost.  That mechanic is already in Fallen Enchantress to an extent with the cloak that allows a champion to cast Blindness and being able to pick spell schools for Henchmen.

I'd like to see an expanded list of items/traits that provide the unit a cast of a spell.  It doesn't have to be every spell (though that would be awesome), but at least a selection of some of the core spells like Heal, Drain Life, and Fireball would be wonderful.