I may have a suggestion for this ... even though I'm not too familiar with the whole universe of this game and full extent since I cant find a proper group to try it but I have an innovative suggestion .....
if the players are the ones creating the world and story, allow them to control the magic as well, a mage charecter would be capable of of "researching magic" where he would custom create a spell, from runes that would be compiled into an ever growing list, mixing and matching witch would lead to different results where the magic class's would all count as mystics, and depend on what spells the players research through they're journeys... finding runes in towns, chapel's, dungeons old castles and ancient documents(drops)... with mixtures of runes ranging from blessings to curses and from minor fire to majestic cataclysmic powers of nature ... the use of runes would remain limited at the beginning and it would grant content that would unlock through game play experience in mixing and hunting down every rune.... also leveling up would allow a growing use of the runes, from 2-12 where each additional mix would create additional effect ...
Simple magic would be permitted for all class's such as a light spell or something that insignificant but proper utilization of the runes would be restricted to magic using class's .... with that the players would have a reason to search the whole world for symbols that would allow the use of magic...