Digging a little deeper into the syntactic magic system from GURPS Thaumatology, I came across Realms and Power (p. 188). This system seems to be the closest starting point to what I want my magic system to look like. In this system, each realm has an IQ/VH skill that determines the caster's power and their ability to control magic. Each realm is then divided into between 3 and 6 (max recommended) levels that determine which verbs are available to the caster.
The system I discussed in my last post proposed an affinity system. Each realm (or in my system, color) would have a level governing how powerful the caster's magic is, while there would be a skill for each color. The skill would be heavily penalized, but would cover every verb. Individual verbs could be bought up as techniques, with more specific verbs giving less of a penalty to cast.
As a worked example (and I'm just making up numbers at the moment): A wizard with red affinity wants to cast an explosive fireball spell. He could cast the spell at his realm skill at, say, -10 to skill. If he had a 15 skill, he'd be rolling against a 5. Using the verb "project", he could cast the spell, at say -5, bringing his casting level to 10. Specializing even further into the verb "bolt", he could cast the spell at maybe -3, bringing casting level to 12. Finally, with the verb "explosive ball", he could cast the spell at no penalty and cast at the full 15.
The radius of the explosive fireball would be determined by how much energy he pumped into the spell. The amount of energy that can be woven into the spell would be dependent on his Power attribute plus his levels in the red affinity. This part is going to take some math to figure out...I can't wait!
So conceptually, I think I've settled on this as a system. My realms are pretty clearly laid out in earlier brainstorms, so it's just the verbs and the actual values that I need to work out. I'm still likely to use Threshold Limited Magic, with the added feature that as a wizard racks up threshold points, they take of traits of that color. For example, using violent green magic might temporarily (or in bad cases, permanently) give the wizard the berserk or bloodlust disadvantage.
I should probably add something here.