Well I can play more than one, but we can just let him/her escape and be forced to chase after them, so we can delay that particular bit.

Well it would be fun if it was a set back of sorts, like a curse of sorts, like you want to be a rogue, well due to the magicbond you have you now also have an evil double that holds the same powers and strength as you.

Or in the case of elemental magic a certain corruption spreads through the body every time it is used altering the character.

With healing magic it is costly for the characters body and life force as you take on said wounds/poisons ect.

Light or shadow magic result in the spirit leaving the body, thus the body being an easy target.

Psychic or Mental powers you become blind.

Shapeshifting is straining on the memories of the mind and thus risk of losing one's self.

How's that?