Spells Are The Simple Part

I'm in the process of getting the appropriate Energy Reserve, Power Investiture, and Pact calculated for Clerics and Druids.

Druids can select from Air, Animal, Earth, Fire, Plant and Water colleges.  Still have to pay for each spell like a Mage would, but no prerequisites.  Their sanctity level is based on how close to nature their surrounding are.  I remembered I have GURPS: Celtic Myths and they have a much more detailed, and better, setup for Druid magic than what I was coming up with trying to make things fit.  Different prerequisites and paths based on what wood is associated with the spells.

Clerics get to choose from Communication & Empathy, Food, Healing, Knowledge, Light & Darkness, Necromantic and Protection & Warding colleges.  Their sanctity levels are based on how holy the area is to their chosen deity.  The only place they would have no power is in the consecrated ground of a deity diametrically opposed to theirs.  Don't stand on the alter of your god's enemies and expect your spells to work unless you do some serious desecration first!

Illusionists are simply one-college only mages using the Illusion & Creation college.

Yes, this means that a Mage can do everything that the other three kind of spellcasters can do.

But...

Clerics don't have material components to deal with.  There will be a penalty for NOT using their holy symbol.

Druids don't need material components, but will get a boost if they use their holy plants as part of the evocation.

Something I'm changing from the main GURPS rules about spells is putting a price on material components rather than just assuming the Mage covered it and they're negligible.  On the Appendix: Spell Table starting on Magic p. 223 there's a "prerequisite count".  That number, squared, is the G$ cost of the materials needed to cast the spell, times the effect.

For example:  Fireball has a prerequisite count of  4.  So a 1d fireball would cost G$16 to cast in addition to the 1 FP.  2s is G$32 and 2 FP and so on.  Force Dome is an area spell, so it's prerequisite count of 11 means G$121 for a 1 hex radius, G$242 for a 2 hex and so on.

It's coalescing.

Finally!