Long post with my ideas:
I think their should be diffrent types of magic, obtained in different ways and with different effect
Runemaster: A glorified craftsdwarf, able to create runes from "higher" stones and attach them to weapons or other objects for bonuses.
Brimstone rune will cause weapon to do additional fire damage, granite rune improves armor, onyx rune decreases wear on items. Dwarves will obviously like runes that are of the same type as their favorite mineral.
Druid: An elven noble, in order to get him/her you need to have very good relations to the elves and not cut any trees for X seasons. This noble will cast spells on crops and trees to make them grow faster and last longer before they wither. Also he shapeshift trees into furniture without killing them first. Woodcutters and carpenters get unhappy thoughts from seeing him and vica versa. Trading elves love "natural" items but still don't like "unnatural" items. In case of emergency he can animate a tree as warrior to fight during a siege(LOTR style ents). After the battle is over the druid is exhaused and the tree goes back to being a tree.
Shaman: The spiritual version of a medic, shamans(normal dwarf with shamanism job enabled) will try to heal wounded using a variery of herbs(treated dwarves heal in X% of the time needed for normal dwarves, X is lower if the shamans skill is higher) and enables the growing of said herbs, is able to brew special drinks that enhance the abilities of your dwarves and is able to cure insane dwarves by getting rid of the spirits possesing them. In a fight their abilities consist out of some kind of war chant, causing all nearby dwarves to go in a martial trance or enrage and a limited ammount of weapon/armor skill. Can also make special totems that give a bonus to the carrier. Totems go well with runes.
Geomancer: A noble obtained by mining out alot of rock. high master mason and miner to start with, the geomancer features a limited "cave vision" that allows him to see further in the dark then normal dwarves. Also can go in a trance to "see" what lies beyond the rock face. in close range like 4-5 squares away from the wall he is able to pinpoint the exact type of mineral but after that it's limited to descriptions like "stone" "ore" "a mineral" "something unknown" and the readings will become more innacurate(about 20% off) further away from theb geomancer. The geomancer is able to craft special suports that combined with a rune from the runemaster are alot harder to destroy and have a chance to survive a cave in caused by retracting them. In combat the geomancer uses his pickaxe and can generate 1x1 stun+injure cave ins and 3x3 knockdown earthquakes at the cost of a few local boulders.
warmage: A human immgrant noble that will come to your fortress if you are rich enough. During times of peace he will generaly be completely useless, hoarding magic trinkets, runes and gems like no tomorow but when enemies arrive he will take his pile of toys to the battlefield, extract the power out of them and use it against enemies. Comes standard with a runed shortsword, runed shield and a set of plate armor. Can throw fireballs at slightly lower speed then sprits of fire by using red objects and freeze enemies using blue ones etc. The archmage will go uncountious from over-exertion by casting too much and needs more sleep then normal people.
Necromancer: The hardest one to get and keep but also the most potent one. Requirements: 1 succesful fell mood, 100 casualties and the philosopher. Dwarves and outsiders both get unhappy thoughts from seeing him. Upon arriving he will choose 3 dwarves as his acolytes. Acolytes behave like normal dwarves most of the time but when the necromancer needs them they will quit what they are doing and help him. Because of their unhealthy job acolytes and the necromancer wound recovery takes twice as long as normal and they can't be healed by the shaman. However, they don't care about their injuries during their work and contignue to do it no matter how mangled they are. Everybody else will get very unhappy thoughts of seeing them, especialy the shaman. The necromancer can perform a summoning ritual, requiring alot of bones, the presence of his helpers and a target to raise. The ritual is very taxing on the casters and inflicts alot of wounds on them. Depending on what they summon the wounds are more/less severe. The necromancer is also able to do civilian work but wont join the military. The acolytes can join the army but will quit as soon as the necromancer calls them. Summoning rituals cause alot of noise and normal dwarves seeing it will get very unhappy. As soon as you get the necromancer the druid will quit and the elves start a war.
Bone construct: Can be created from bones and a skull. Very dumb and slow hauler. Hardly any damage to summoner.
Skeletal <Type>: Can be created from any remains of the samy type as the target
Dead elephant --> skeletal elephant
Dead dwarf --> skeletal dwarf
Smarter and faster then the construct but more taxing on the summoner
Zombie: summoned the same way as skeleton, alot slower but stronger.
Ghost: Requires nothing to be summoned, invisible scout with stealth detection that can walk through walls(steal this, goblin b*tch). Moderatly taxing on the summoner.
Shade: Advanced version of the ghost, but requires one dead acolyte(if none available, one of the current acolytes will be sacrificed and the necromancer chooses a new one. Can haul items through walls. Very taxing on the summoners.