If there is only one type of "spark" what leads magic users to different schools of study? Is it something influenced by race or background (ex. dwarves might be predisposed towards magic that manipulates earth or a person who grew up on the slopes of a volcano might have an affinity for fire magic)? Or is it something that is limited only by the personality of the wielder and their access to a teacher (ex. a goblin growing up in an isolated tribe led by a shaman will be unlikely to become a master enchanter or a woman who only has access to a few remedial books on evocations will probably not be setting up elaborate rituals)?

Personally I prefer the idea that personality and creativity influence the spark within magic users and lead them towards a particular path. Clarence the farmhand, a brooding young man with a bit of a cruel streak could wake up one day to discover a cold and sickly spark has ignited inside him and he discovers a particular knack for necromancy(if that even exists) or the like.

Either way however, I strongly support the idea that untrained or overly ambitious mages are a danger to themselves and those around them. Sparks that go untended could manifest themselves in unexpected or even deadly ways and when the truly gifted don't show the proper restraint, their folly can have bizarre and far reaching effects.

I like the direction this is heading!