I think the way it would be implemented would be just a new ability with an extremely long cooldown like the 2 hour abilities (well, 1 hour now) from Final Fantasy XI. Make it the longest cooldown, but a very powerful ability/transformation that can be used say once every 10 minutes. Like our current 5 minute CDs but better and longer.

Like Monk could get 100 fists (reduce GCD time further and increase auto attack rate for a few seconds), Warrior can get Mighty Strikes (crits for the next X seconds), Black Mage could get Manafront (no MP use for a few seconds for all the flares), White Mage could get an AoE Benediction, Dragoon could jump multiple times (like Freya in IX), etc.

These abilities would likely have to be saved for DPS Checks and whatnot, and all would need to be properly balanced which would certainly be a pain (Monks/BLMs only cause they get best DPS boost! Who cares about LB!?). And it would probably be the last (or possibly only) ability we get in 4.0 (since people are complaining we have too many abilities already)