just my 2 cents, but how about something like this?
NPC masters scattered around the world and one for each class, once you talk to one of them you can then pay to have them teach you their ways on one condition...that you must fully dedicate yourself in the new discipline or else the master will think teaching you his/her skills a waste. also such dedication means that your primary discipline cannot be realized to its full potential, for each level you train with the master is a level that you cannot gain anymore for your primary class.
ex: level 5 paladin trains with fighter master, gains 1 level of fighter (if the level cap is let's say...30, then the paladin can only level to 29 if he has 1 fighter level in order to always keep the sum of the levels equal to the level cap)
ps: first post in this forum so please forgive me if i'm not aware of some of the game mechanics like the levelling system.