The rule here is to add in abilities that could actually work AND are classic FF abilities! Have fun! Post incoming.
Paladin: Dia--deals AoE damage and AoE dispel. Costs a metric ton of MP. This will make it so that dispel can actually be used, and monk won't be the only job capable of doing so. Would be a godsend in PVP as well, and fits the tanking role.
Warrior: Mad Rush--increases skill speed by 20% for 20 seconds. Grants one stack of wrath/abandon. Becomes pacified for 5 seconds after use. 90 second cooldown.
Dark Knight: Doom--kills an enemy in 30 seconds and below 25% health. If enemy is immune to death, doom reduces the enemy's strength/dexterity/intelligence/mind by 10% for 10 seconds. Stacks with virus and other such abilities. 150 second cooldown.
White Mage: SOS Wall--places a buff on an ally that allows ally to take 20% less damage from all attacks for 10 seconds if their HP falls within 25% in the next 10 seconds. If HP does not fall to within 25% in those 10 seconds the effect wears off. 1.5 second cast. MP cost similar to Esuna. I know, it seems fairly useless. Why not just get a cure off? And it's not like that 20% less damage will save them if they're at a quarter of HP. But I think the best players in the world might find a place to utilize this, such as letting the HP fall, the co healer gets the tank topped off after the buff activates before a big tank buster, and tons of damage ends up being mitigated.
Scholar: Scan--enemy takes 1.25x more damage from critical attacks, and cannot deal critical attacks for 10 seconds. Costs one aetherflow stack. Scholars HAVE ALWAYS HAD SCAN. IT IS THEIR THING. This iteration of it makes perfect sense. It displays a weakness (allowing allies to deal more damage), while predicting their next move (therefore preventing enemy from using a critical attack).
Astrologian: Haste--increases an ally's skill speed and spell speed for 20 seconds. Healing DoTs on that ally tick twice as often. Costs about as much MP as benefic 2. 2.5 second cast. The expensive cost and fact that it can be used on one ally means it won't be spammed; instead, it'll fit quite nicely into the Astro's healing rotation.
Summoner: Call--replaces current egi (or summons one) with a randomly selected one for no MP cost. Upon being summoned, the egi does its most powerful attack (which doesn't go on cooldown). 5 minute cooldown.
Black Mage: Drain--deals damage and restore HP equal to damage dealt. Potency can be higher than scathe but not by too much. Can proc to be used again at no MP cost and no cast time. MP cost about equal to scathe, 2.5 second recast.
Bard: Hero's Rime--all allies in range take 5% less damage. Deals 15% less damage while singing. Drains MP while singing. Can be used with battle voice.
Machinist: Mimic Daravon--deals decent AoE damage and places a Debuff on them so that when they attack an ally they are induced with sleep status. 1 minute cooldown.
Dragoon: High Jump--deals decent damage (equal to power surged jump) and avoids damage while in air. 1'second spent in air.
Ninja: Shadow Pin--deals small damage and binds an enemy. Used with a mudra combination. While bound, enemy only takes critical hits.
Monk: Focus--use remaining chakra stacks (has to be four or less) to increase potency of next weaponskill by 50% each time.