Here's all the known bosses for Siege of Org, including some pics and fight overviews. Let the speculation on further bosses begin!
Siege of Orgrimmar Bosses
Part 1 - The Ruined Vale of Eternal Blossoms
Immerseus - Tears of the Vale
Giant sha-infected water elemental.
The Fallen Protectors
Pandaren members of the Golden Lotus, who are all bad and stuff.
Mogu-shaped titan construct that formerly watched over the Dark Heart of Y'Shaarj before it was taken by Garrosh's forces.
The Sha of Pride
The Seventh Sha
Part 2 - The Siege of Orgrimmar
Galakras - Zaela's protodrake mount
A big protodragon with some kickass armor on it. Untextured image only so far.
The Iron Juggernaut
The Horde's newest warmachine.
Kor'Kron Dark Shaman
A pair of dark shaman named Haromm and Kardris. They summoned debased and polluted elementals.
Part 3 - The Siege Continues
Boo, hiss. He probably goes all Old God corruption on us, but no pics of that yet.
Spoils of Pandaria
No idea what this one's about - sounds like a non-standard encounter. Could be enslaved Pandarian races/beasts, captured Mogu tech, etc. No sign of Aethas Sunreaver as a boss encounter yet, so he might be here with mogu Blood Golems and so forth.
Thok the Bloodthirsty
Some guy named Thok. Apparently he's the violent sort.
A goblin, I'm betting. Guess some of them stand by Garrosh, at least.
Part 4 - The Heart of Darkness
Paragons of the Klaxxi
"Your gods are not our gods, outsider. If the Old Ones ever return, we mantid will once again stand by their side." - Kil'ruk the Wind-Reaver
Garrosh Hellscream - Warchief of the True Horde
You've come a wrong way, baby.
edit - and heeere's his new toy:
The Dark Heart of Y'Shaarj.
So far there is word of several world bosses - the four August Celestials and a yaungol who has become a demigod through apparent elemental ascension. The four celestials are probably all one encounter.
The August Celestials are encountered on a new small zome called the Timeless Isle. Orods' whereabouts are unknown at present.
source for much of this info - http://leviathonlx.blogspot.ro/The Celestial's are World Bosses on Timeless Isle.
Dungeon Journal entries:
Chi-Ji, The Red Crane: The trial of Chi-Ji is a test of hope. Uncertain of the future of his people, Emperor Shaohao sought out the great Red Crane. The Great Crane challenged Shaohao to find the hope within himself, and cast off his uncertainty. Once the Sha of Doubt was vanquished, the Emperor became a beacon of hope for his people. To pass the trial of the Crane, warriors must believe in hope, even in the darkest of days.
Yu'lon, The Jade Serpent: The trial of Yu'lon is a test of wisdom and faith. With omens of a bleak future floating through his mind, Emperor Shaohao climbed to the summit of Mount Neverest to seek the council of the Jade Serpent. Yu'lon urged Shaohao to cast off the dark emotions that plagued him, and through his trials, Emperor Shaohao would become one with the land. To pass the trial of the Serpent, warriors must be wise in their choices.
Niuzao, The Black Ox: The trial of Niuzao is a test of bravery and fortitude. When Emperor Shaohao was overcome with fear, the Black Ox counseled him to face his fears directly. After a protracted battle with the Sha of Fear, the Emperor emerged victorious. To pass the trial of the Ox, warriors must be resolute in the face of overwhelming danger.
Xuen, The White Tiger: The trial of Xuen is simultaneously a test of strength and a test of restraint. When Xuen gave Emperor Shaohao a 10-foot-pole and asked him to strike one of Xuen's warriors, the Emperor could not land a blow. For without restraint, anger and hatred run wild. To pass the trial of the Tiger, warriors must harness their power, while burying their thoughts of violence.
Ordos, Fire-God of the Yaungol: Ordos was once a shaman of the Yaungol who sacrificed himself to the flame, following the promises of the Fire Lords, in order to become a demigod. His flesh is eternally seared with fires, putting him in endless agony. Yaungol Fire-Keepers ritualistically set themselves ablaze to emulate the ascension of Ordos, and cast enchantments and prayers to ease his pain.