Ideally they would expand the soul crystals so that you could toggle between roles also limiting the types of spells you can use and equip for each along with gear set-ups and stat bonuses from the crystal - this is the only way I can see them implementing multiple role jobs other wise it would just be a DPS with some support skills.

In terms of gathering skills I think they are pushing to having you get more skills and traits through quests anyway so that seems like a none issue, unless you really really want to complicate it and have every single mob in the game have a skill you need to steal off them and at that point I wish you well with your blue mage journey because I have no desire to do that what so ever.