The idea is really simple, after completing a quest before talking to the NPC you can simply just change your job and receive SP on any job that you wish. This idea is something that was implemented in Valkyrie profile where after completion of quests or other things you receive bonus Sp which is then placed into a bank that you can allocate to any of your characters. This would be the same type of idea however a bit expanded on. sometimes you dont want to allocate all the SP from a quest to a single job but many in order to gain levels that you are close to attaining. it can be further expanded upon by implementing a merit system like FFXI where after a certain amount of xp after level cap you can get stat bonus's ect... I think this would be a good idea because not only would it motivate people to actually do the quests but it would also motivate players to help out when later high level content is put in place.

a few way to make it so the system is not abused... you cant just grind on a 50 character and level up all the other characters without playing would be adding a separate type of SP you gain when meriting or implement a limit to how much SP a class can be allocated in its lifetime. say 100k

This idea is raw and open to criticism.. k thanks.