It would be very easy (in my opinion) to make them a full job.
- Create spell list #6 which is strictly for group play. Some spells will either need to be forbidden from being slotted and will be greyed out, or will need to have their effect changed to set damage, support, or mitigation (like Missile, Doom and Mighty Guard).
- Set #6 will only work if it has a total amount of potency or power. If you don't meet X amount of potency/power, you can't queue; just another check along with ilvl. Also include a max value, in a similar concept like TT card decks.
- If you only meet the minimum potency, it will be due to having several support skills like White Wind equipped. Make this minimum potency equal to RDM DPS.
- If you hit the maximum potency, it will be due to having no support skills equipped. Make this maximum potency equal to BLM DPS.
- Increase MP cost of White Wind and Bad Breath when placed in list #6. This will keep their usage limited to emergencies special damage mitigation.
- Spells that paralyze and petrify can actually stay. They are functionally no different from a WHM Holy stunning trash packs, a BLM sleeping groups in low level dungeons, or a PLD pacifying a boss. If an enemy is immune to Sleep, make it immune to paralyze (seems to be the case with bosses already). If an enemy is immune to stuns, make them immune to petrify.
Since we are limited to only setting 24 skills as it is, it should be very easy to balance. It would echo FFXI's system of each spell being worth so many points. With the types of spells BLU gets in this iteration, it wouldn't likely be very cookie-cutter, either. You'll have some BLUs that prefer to focus on some support via various enfeebles and WW who pull the DPS of a RDM, you'll have some BLUs that prefer to go all out with damaging spells who pull the DPS of a BLM, and you'll have some BLUs that fall somewhere in the middle pulling the DPS of a SMN.
The one thing I'm really unsure of what to do with, would be the spammability. As it is, you pick your best potency spell and spam it. Perhaps list #6 should increase the cooldown on all spells by a lot, with each cast reducing that cooldown in a way similar to SCH's Aetherflow cooldown being made shorter with each AF ability use. This way you could cast your long range spells in any order you wish and when they go on cooldown, Loom or Flying Frenzy yourself to the enemy to cast your short range/breath spells which in turn will make your other spells cooldowns come up more quickly. Assuming you select breath spells, with future levels it could be made possible to have all long-range spells probably.
One issue I do see with it would be when paired with RDM. Peculiar would buff both the BLU and RDM, and RDM's buff works on BLU's physical spells. So it might turn into always trying to pair those two. The other side of this, however, is that this is already seen with DRG/BRD, AST/SCH and to a lesser extent with most melee DPS and tanks.