One of the systems I found really fun & engaging was the player training system in star wars galaxies.

"Star Wars™ Galaxies also offers players the opportunity to teach one another certain skills. In fact, the concept of apprenticeship is key for advancement in some professions.

Some skills require Apprenticeship XP. In order for another player to be trained they must be grouped with another character. You can then teach that character specific skills that you possess. The other character needs to have any prerequisites, including XP, for the skill you want to teach.

Becoming another player' s apprentice has several advantages. First, you can learn any skills that another player possesses without seeking out an NPC skill trainer (thus allowing you to train in the field). Second, NPCs will nearly always charge for training, but you aren' t automatically charged credits when you learn a skill from another player."

While I think the council sink of NPC trainers is a good system it would be really neat if there was an alternate path. Maybe needs research like the ice/fire magic, maybe it's only skills not the cap unlock and require a skill book too.