Make a small change that would break a class completely.

[FenreyVarnell]

and for the three new jobs for fun:

Dark Knight: When HP falls to 1, [insert passive here] will restore all your health and grant you "Berserker Armor" for 15 seconds. (take notes, Yoshi-P.)
Astrologian: Receives the limit break "Lunar Whale."
Machinist: When loading your cannon, your Turret will now distract a random target by telling them: "These are not the Turrets you're looking for."

[Nutz]

I'm not sure people got the 'small change' memo >_>

In any case, in typical FF fashion: healing spells now damage undead. Raise instantly KOs and undead target.

[Gilraen]

I don't think anyone truly understands what 'break' means, either. Or that the term 'class' was specifically mentioned instead of 'Job'. I think I've been pretty spot on with my ideas.

[DarkStar]

MNK: Greased Lightning now stacks up to 5 and lasts 10 seconds longer.

PLD: Sword Oath now removes Bonus Emnity from abilities and applies its Bonus Damage to abilities as well as Auto-Attacks.

WAR: Crits now apply a stack of Wrath.

DRG: Dragoons are invincible while using Jump, Spineshatter Dive, or Dragonfire Dive, and those abilities deal 25% more damage.

BRD: Bard Songs no longer impose a DPS penalty.

NIN: Ninjutsu cooldown has been lowered by 10 seconds.

WHM: New trait that restores 10% of total MP on critical casts.

BLM: Astral Fire no longer prevents MP restoration.

SMN: New Trait added that increases Critical Chance by 25% of Spell Speed.

SCH: Can now have both fairies summoned at the same time; the most recently summoned is the one directly controlled by the SMN.

[Sove92]

Some more OP, some less, but still break balance.

WAR: Berserk no longer inflicts Pacify after the effect ends. Fracture TP cost has been reduced to 70. Storm's Eye now stacks with Dancing Edge.

PLD: Shield Bash has is now off the GCD but still costs 150TP. Fracture 70 TP

BLM: Manawall and Manaward can be cast on party members.

SMN: Bane spread is no longer limited to 3 targets.

WHM: Regen and Medica II ticks no longer generate hate.

SCH: Crit Succor now doubles the shields. Whispering Dawn no longer generates hate.

DRG: Jumps have invincibility frames.

MNK: GL lasts longer.

BRD: Ballad/Paeon no longer lowers DPS.

NIN: Dancing Edge stacks with Storm's Eye. Mudra sequence is no longer interrupted by other skills.

[reillyblair]

thm gets the spell at level 60 to summon meteor to obliterate eorzea

[EdgeFayth]

Bard : Mana song also affect the bard

My favorite

[Felis]

For better skill animations animation-lock come back

[Seif]

Dragoons jump actually jumping out of harms way. (you know like the point of that move has always been since forever)

[Raminax]

I don't think anyone truly understands what 'break' means, either. Or that the term 'class' was specifically mentioned instead of 'Job'. I think I've been pretty spot on with my ideas.

Well, even in the OP's case, they use a Job exclusive spell...

On topic:

Life Surge is no longer consumed by the next skill used. 15 seconds of guaranteed crits, woo!
Esuna/Leeches no longer have any restrictions on which debuffs can be removed.
Aetherflow stacks are now infinite, eternal, imperishable, endless.

Yeah I dunno D: