To facilitate more roleplaying aspects of the game and to introduce more important NPCs, I'd like to add training to the "level up" process. Whenever a character gains a new level that offers new special abilities (including spells), the character must work with a trainer to access these new abilities. Increases in HPs, BaB, and saving throws can be gained without training.
Are you going to require a cost or will that be a roleplayed item dependent on the trainer? Kind of like the latter- perhaps a wizard's mentor doesn't charge but he does expect the apprentice to perform errands. Or maybe most mentors will either expect a gift (a map, a new spell, a good tale (for a bard), or perhaps money). If they don't get such a gift, they may ask for something explicitly or an errand.
Also allows for 'new trainers'. Perhaps if you are too far from your trainer, you find one in a city but since you don't have a relationship with him, you just give him money whereas if you travel to your old trainer, he would like be much more flexible (or require less money).
I was planning on a trainer-by-trainer / pupil-by-pupil basis. You show up at a trainer and seek training … the trainer lets you know what he requires … maybe it's free, maybe it requires an errand, maybe he just needs some cash.