I just had a crazy idea. You know how sometimes big online games like Ultima Online have lots of skills you can learn? Why not have two stats. Knowledge and Skill. The higher your knowledge level, the faster the skill goes up when you use it. To raise knowledge level, you have to go to some guildmaster or npc sage or some such and take a test. The test would be based on real world information. For example, to raise blacksmith knowledge, you take a test derived from medieval blacksmith history books and such. Well, just another thought.