The Bastion of Twilight

It is confirmed that Cho'Gall will be the final boss of Bastion of Twilight.

Could it Possibly be? Cho'Gall itself
The first of the Ogre Magi

No. He won't be a hardmode boss. Instead it will work like this: You kill Cho'gall on normal mode (either 10 or 25 man). then you can be able to progress to the heroic mode. It is then you expect him to be probably much more difficult than his normal counterpart. But wait there's more! Once you kill Cho'gall, then an optional hard-mode boss becomes accessible to your raid: Lady Sinesthra. Blizzard has stated that she'll be Cataclysm's first tier of raiding's "endboss", sort of like how Algalon was during Ulduar. She'll probably be on the same level as Algalon in terms of difficulty and the gea

Are you sure it's Lady Sinestra? The only reason I ask is lore-wise she was destroyed when Dargonax's power became unstable, killing them both. Only Blizzard would do something so ridiculous as bringing back somebody that was literally blasted to smithereens I guess : /