I do not know if you have already thought this but in my opinion you could create a class mostly based on the combination of Vacuum + Vortex. Imagine a class which can not heal with pots neither it can regain mana with pots.
You will be forced to balance your use of Vacuum which will drain fast your mana to heal your self, while you drain that life from enemies, with the use of Vortex which will replenish your mana but in this hypothetical scenario should not cause damage to the enemies. Just a defense buff to you.
It will be up to the player to decide when to stop leaching life in order not to stay out of mana. He will have to quickly switch to Vortex which will not harm his enemies but it will give him some more magical power to continue with other spells.
