[Autochthon]

CC spells would be nice - there's a TT game called Warmachine where some mages can make or break a game that way (The Kreoss 'Drop and Pop' Feat comes to mind). Either PBAoE short-term root/Knockdown/Stun or a Snare/Kockback come to mind.

Conversely, an escape spell that temporarily removes the caster from combat but also makes them unable to be attacked. Another version would be one that safely returns the caster from the battlefield to the nearest lab when hit with an attack that would otherwise have killed him (Imminent Death or delayed action spell triggers?).

Fear or Autorout spells - essentially, if the target fails their MR and Morale Check, they automatically flee. Period.

Chain Lightning/Cleave - essentially an effect that hits the first unit in a square with the full effect, and the next with 66% and the last with 33%.

Blood for the Blood God! - Blood effects that benefit not only from sacrificing Blood Slaves, but also based off of the amount of blood is shed, total damage inflicted that turn or units killed up to that point. This wold make for some interesting tactics, such as suiciding chaff to power spells.

Irresistable Missiles - Some spells deliver tremendous momentum to their magic missiles, and can slam through packed masses of troops like they were not even there. Give such high-energy projectiles the equivalent of Trample.

Path Synergies/Resistances - If a Mage casts a single Path spell, but also has skills in a Path that is considered synergistic, then the spell's effect/penentration is increased.

Conversely, a Mage with a particular skill in a Path or Resistance also gets an innate MR vs. Paths considered antagonistic to that Path.

Complicated, I know - but you did ask

If this sounds like stuff from MMORPGs, you'd be right