Elemental is a fantasy-themed strategy game. As such, I expect units to not only use brute force, but also to use magic. The idea was implemented both in MoM and in HoM&M (Heroes of Might & Magic). Neither of them did it well. My idea goes like this:
I want to train mages (magicians) to bolster my army. For that, I need a high level specialized infrastracture (wizard guild). This is both simple and common. The hard part comes when implementing said casters in combat situations, where you want them to actualy behave like casters and not like archers.
Bad example, MoM's mages: unit of either 4 or 6 individuals (depands on race), have a specific attack power (depands on experiance) and have a one-time spell (very weak fireball). These guys are just over powered mages (they don't get to-hit penalties from range)
Decent example, HoM&M4 Efreet: have a small mana pull and small fire-based spell repertuire. This is good for a half-caster, but it's not a mage. It's annoying that HoM&M doesn't have multi-porpuse caster units.
Ideal: I train my mages to be able to use specific spells (same as I train cavalry to charge or pikemen to hold formation), then in combat, I can order them individualy to cast specific spells, or to order a group to cast a single spell together.
For example, I would like a single mage to hold an anti-arrows barrier while the other four are busy summoning a fire elemental. When they finish this, I want to be able to tell two of them to cast a defensive spell on my infantry while the other three are trying to hold off the enemy's cavalry with a firewall.
These casters should be a standard mid to end-game unit. They should have a decent amount of mana (I.E. ammunition), enough to cast at least a few spells, and also to have some way to regenerate mana during really long fights. This kind of unit has incredible potential at the hands of a skilled/creative player, and hey, what's more fun than watching your battle-mages torch an enemy unit with fireballs?