Recently I was watching Magi on Netflix and was reminded of the wild magic used by elves in The Hallows series by Kim Harrison. Both settings have magic as a living thing, with the Hollows wild magic is little eyes that are avatars of a goddess and in the Magi series magic is based off souls and energy that comes from them.
Here is an idea for duplicating that concept.
Spirit Ally with Injury Tolerance: Diffuse! (Magic, -10%) 
This allows the spirit to do various powers with the special effect that it envelopes or flows over the subject. This typically includes elemental shaping but not creation effects as it can condense water or move air. The power must be less than 360 points and have the Magic PM, typically Affects Substantial, No Range or Melee based are included in the modifiers.
Mages then summon the ally to do their bidding by giving it specific instructions. In both settings the base are nonsapient so orders typically require a Concentrate (fitting for magic) and abilities are really limited only by imagination.
Perhaps different effects each require a skill roll to achieve, except for alternative abilities representing learned commands (much like Learned Prayers in Divine Favor or Sorcery).
Obviously this could be just a special effect for regular power builds or perhaps an alternative to an existing spell casting system. In that case perhaps replace the typical Magic power modifier with a variant that allows Affects Insubstantial attacks to counter the spells. Essentially a dispel magic is shattering the cohesion of the spirit, or perhaps even killing it.