The main issue i see, is that Blu in a MMO (all i have to go off of is XI) but Blu is all about diversity and adapting to the situation. It worked so well in 11 because of the variety of situations you could find yourself in. Elements mattered, Resistances mattered. These are 2 very key things that simply don't and can't exist in this game the way they did in 11.
It would require a complete overhaul of the game. Casters would need a variety of elements to compensate for the monsters resistances, which points back towards too many abilities and spells for hotbars.
In XI if you were a monk, going up against something with blunt resistance, you could equip a slashing weapon, and for the most part, there was at least one weapon in every type that was a different attack style, Joyeuse being piercing instead of slashing for example. In 14, Monk gets H2H weapons, which are blunt. That's it. So if they introduced content that had only Blunt damage resistant monsters, Monk would be banned from it by the community. Likewise if a monster had immunity to Fire damage, like Ifrit, Blm is obsolete.
Blu in 11 was all about monopolizing on the situation, so a monster weak to fire, or piercing, you could set spells to exploit that weakness. Right now, Mountain buster and the lightning counterpart, its quite pointless. There really is no reason to get both aside from completion. (yes, i know the Blu exclusive content has a use, talking outside of that)
They made 49 spells, and like people keep saying, most are garbage, or won't be used. Bristle would be Great if it can be used with the Primal abilities, but from what i am gathering, it can't. If it was insta cast, AND could be used for more than 1 spell, it would be worthwhile, otherwise its a wasted spell. Any of the lower potency various elemental spells are pointless.
If they had more varied ranges, cast times and costs, that would be a start.
I still like the idea of having a quest that unlocks the limited nature of the job, but also revamps a lot of its abilities. Meaning for the Blu exclusive, you could use Blu as it stands now. And for actual content, it would be modified.
And also the comments about jobs being clones of each other, they kind of are. All tanks have damage taken cooldowns/shield proc/parry/block, All heals are almost mirrored with a few unique abilities. Ast gets Holy, just in 2 parts. Rdm and Blm play different, but in a game like this, where there are assigned roles, they are DPS. All DPS essentially do the same thing. Thats the one major issue with this game, is a job needs to fit into a Role, which already limits what it can do. And then the other jobs in the role need to be balanced, otherwise there will be preferences.