We basically already have burst in thunder 4.
Dark is used exclusively by NPC's in this game, meaning it could see life as a BLU spell.
Comet is essentially the Mage LB 2.
WHMs should probably be able to cast water, but they'd need to find a way to make it fit in with the rotation.
Quake is used by NPC's a lot as well, but it would probably end up a WHM spell here due to the elemental affinities of the jobs (I don't think BLMs can use Earth Aether, I'd have to check), Tornado too would be WHM.