In FFXI, each class had one ability that had a two hour cooldown (later reduced to 1 hour) and had a very powerful effect. I'm hardly asking for FFXI-2 here, but this is one aspect of it that appealed to me. Of course, considering the fast pace of this game they can probably work with a 20 minute cooldown, enough to ensure that it's only seeing use once, maybe twice per dungeon. Maybe something like this as level 60 skills:
Paladin: Shining Sword - Provoke all enemies in a player-centered AoE, generate additional enmity equal to Rage of Halone, and reduce their accuracy by 90% for 10 seconds.
Warrior: Primordial Fury - Instantly grants maximum Wrath and Infuriated. You remain Infuriated for the next 10 seconds no matter what you do.
Monk: Thousand Fists - Instantly grants maximum Greased Lightning. For the next 5 seconds your auto-attacks will have their rate tripled, and the GCD will be halved.
Dragoon: Rising Dragon - Dragoon jumps, losing any heavy or bind effects, and remains in the air for 3 seconds. During that time he can still move but cannot be targeted or damaged by anything. Upon landing, all nearby enemies take a 500 potency hit.
Ninja: Kage Bunshin - Ninja summons a clone that will mimic every move the Ninja makes. Clone's actions have half the potency of the original. Clone loses 10% of it's HP every tick (dies in 30 secs if it receives no additional damage) and cannot be healed.
Bard: Encore - Bard uses every song at once at double effectiveness (doesn't stack with Battle Voice) without losing DPS. This includes Swiftsong if out of battle, but it's effectiveness is unaffected.
Black Mage: Manafont - For 10 seconds, the BLM can cast spells even if they do not have the required amount of MP; in such a case, the spell will instead consume all their MP, or none if their MP is alreday 0. Casting Flare will end the effect immediately.
Summoner: Grand Summon - For 20 seconds, all Summon abilities have 0 cast time. You may have up to 3 Egi out at once, however they can't be controlled. Rouse, Spur and Enkindle apply to every Egi, but Sustain only applies to the most recently summoned one. When the time runs out, all but the most recently summoned Egi will vanish.
White Mage: Arise - Raise a target with full HP, MP and TP, and without weakness.
Scholar: Fae Blessing - 100% chance to crit for 10 seconds.
I'm no theorycrafter, so I don't know how balanced any of these would be.I'd also like to know if the general idea behind it is sound.