I kind of think that Classes need to step back if they are going to stay at all. Let them remain being good at casual soloing etc to help the newer players but the Classes should just become basically weapon professions (Which they pretty much already are). They can also remain as pre-requisites for Jobs but ultimately Jobs should be entirely separated from Classes. Instead allow a Job to pick from either all Weapons/Classes or a specified few dependent on the Job. Jobs could gain abilities/traits etc etc from acquiring Job Points(JP) and like what Shougun has been saying unlock new tiers of abilities through the completion of quests etc.

Something like this could be done any number of ways i would imagine.