Project 39: It's a kind of magic
Arcane Magic is classic energy manipulation, making things move or burn. While there won't be any room clearing fireballs, this is the most useful magic for combat.
Spirit Magic is derived from faith, and in fact many users won't even think of it as "magic". It will be healing and protection magic. Always useful to have when combing otherbeast infested ruins.
Druidic Magic (working title) taps into the primal magic that flows between the Otherworld and our world. Most useful for "tweening" between the worlds, affecting otherbeasts/otherkin and affecting nature.
The approach I'm looking at has some resemblance to the Force users in Star Wars Saga. The hero has a feat to qualify, a la Channel Magic, has a specific skill to use a given kind of magic, then uses additional feats to pick up "spells". More powerful spells would likely have lesser spells as prerequisites, such as having Ignite (a minor spell used to light fires) as a prereq for Combust (a combat fire spell useful for setting bandits and ogres on fire).
The hero has their "suite" of spells, each of which may be used once per encounter. Some might take multiples of the same spell to get multiple uses, others might have feats that let them get certain spells returned to their suite under certain conditions during the fight.
The other mechanic I'll be using I call Axiom. It represents the mundane worlds "resistance" to magic. When a spell is generated, a number of Axiom dice are rolled, and the result is subtracted from the skill check. So Ignite might have an Axiom of 1 (roll 1d6) while Combust might have an Axiom of 3 (roll 3d6). Of course there will be feats that modify this mechanic.
The idea is to keep more skilled casters from automatically toasting everything every time and to reflect the difficulty inherent in more advanced spells. Say Willy the Wizard has an Arcane asset of +10 (that is his skill plus all modifiers come to 10). With an average d20 roll of 10 (I know, it's really 10.5), that generates an average of 20. The odds of a target not getting hit by Combust is pretty abysmal given an average Reflex Defense of 10 to 12. Willy has to autofail to miss anyone with a Reflex lower than 14.
Throw in Axiom, and now it becomes more challenging for Willy. Using Combust becomes much dicier, as the average Axiom generated will offset his asset.
I'm still chewing on the terms Druidic and Axiom. I'm staying away from the term Primal Magic because of the Primal Power Source in 4e, but I haven't thought of a better name.