After playing the Beta some, and loving the Chanter lots, I've got some ideas to make some impact in Spell choices or Chants etc. etc.
Undead magic, animation of the dead or anything similar being unlockable somehow through side-quest or main story decision.
Not only is summoning of the dead a morbid act in itself, in the world of Eora/Pillars of Eternity (lore) I recall necromancy being very shunned upon by most groups. This gives more authenticity to the world if some magic is not attainable by just "Leveling up".
Some magic should only be learned by doing certain things and/or find a teacher.
EDIT: Additionally, I made this thread with a Chanter faction idea in mind (although not necessary for the forbidden magic idea): Rattlers of Bone, or Bonesingers. Chanters that sing, worship and praise the dead to dance for them.
Edited by Osvir, 21 August 2014 - 02:51 AM.