Want to make spell casters more exotic? Increase the minimum ability requirements. That’s what I did in my 950 AD alternate history campaign. To gain levels in those classes, the minimum score is 15. If you are using the 4d6 drop the lowest die method of generating characters, the chances are of getting a store of 15 or higher is roughly 23.2%. If your game master allows you to assign ability scores as you like, chances are good that your party will have a Magic User or Cleric in it.
Using the straight up method of 3d6 though, like the NPC population, chances drop to 9.3%, and that makes no guarantee that the 15 or higher stat is in Intelligence or Wisdom. Now also take into account the following factors:
- There may be a literacy requirement before any order of Clerics or Magic Users will admit a student
- There may be limitations based on gender or race
- There may be limits based on age (making it harder for someone to pick up a level later)
If you are running a low magic campaign, its worth it to consider raising the bar.