Ok, so everyone else other than Hunters (and Monks ofc) have their final ability. Our old was was (thankfully) cut, but the new one isn't up yet. So, here are the trends so that we can speculate for ourselves what it may end up being.
1) All the revealed ones are at the very least situationally useful to all specs in both PvP and PvE (Some, such as Shroud of Concealment or Blinding Light, lean heavily one way or another)
2) When their use comes up, all of them have a MAJOR impact on the course of things, ranging from merely abusing the heck out of temporary effects and predictable near-deaths (Mage) to giving and gaining an extremely wide possible spectrum of abilities (Druid) to raid-saving mass temporary CC (Paladin)
3) Notably, none are passive. In all cases you have to hit a button to gain an active effect. These effects are also almost all either instant or short-lived with the sole exception of the Warlock Gate
...not much to go on, but it leads to some interesting thoughts. The old Priest ability amounted to a very fancy personal damage-reduction cooldown with some mobility benefits and a cool graphic potential, and thus violated #2. So, here are some other thoughts on the topic and my personal prediction for the ability.
1) It will NOT be a heal, unless its form and usage changes while in Shadow spec. The current calculators make it abundantly clear that people who aren't directly healers will only have two healing buttons apiece, Shadow is getting Flash Heal and PW:S.
2) It will not be a pure damage ability, due to it also going to two healing-type specs.
3) If it is a buff, it will likely be short-term whether it is Personal or Raid. Replenishment is also right out.
That said, what I am calling for the 87 ability will be a Discipline version of Bloodlust/Heroism/Time Warp. Perhaps not the most original of abilities, but giving it to a third class (and making it castable while in Shadow form!) would ensure that nearly all 5-man groups will have one and simplify balance issues (I remember opening-day Murozond without having one in the party, not fun).
Runner-up in my mind would be a variation on the Shaman's Ascendance ability from the calculator, a personal 15-second buff depending on your spec.