Possible Boss List for Hellfire Citadel

This is purely based on the Mythic Achievements discovered by MMO-Champion.

The names here are in alphabetical order.

Archimonde
Feel Lord Zakuun
Hellfire Assault (Encounter)
Hellfire High Council
Iron Reaver
Kilrogg Deadeye
Kormrok
Mannoroth
Shadow-Lord Iskar
Socrethar the Eternal
Tyrant Velhari
Xhul'horac

Notably this is only twelve bosses listed, and the patch notes mention "The Gorefiend," (which, judging from the model image, is not your original-timeline-grandpa's Teron Gorefiend, but looks more like some hideous abomination.)

Speculation:

I think Archimonde has got to be the final boss here, because no one else is nearly as important or powerful. Mannoroth, even if he weren't a stitched-back-together version of himself, was still subservient to Archimonde and Kil'jaeden. The only person I could imagine outranking Archimonde would be Gul'dan (or KJ - we ain't going to see Sargeras here.)

Actually, Gul'dan's absence from the boss list is the most notable thing here. I could imagine a few scenarios. First is one that I think is plausible but also one I hope does not happen, which would be if Gul'dan were dealt with in a quest outside the raid. Alternatively, Gul'dan survives the expansion. Finally, we might actually get that third raid tier, with Gul'dan as the final boss (though if we're hitting the Legion proper, I doubt that.)

Ok, onto other bosses:

Achievements also divide the raid between the Bastion of Shadows (Shadow-Lord Iskar, Socrethar the Eternal, Tyrant Velhari,) Destructor's Rise (Fel Lord Zakuun, Xhul'horac, Mannoroth,) and The Black Gate (Archimonde - which confirms him as the final boss.) That leaves Hellfire Assault, High Council, and Kilrogg as another wing, which I assume will be the first.

In fact, based on this, I'm thinking (unless it's nonlinear,) we'll go Hellfire Assault, Hellfire High Council, Kilrogg Deadeye (and there's an achievement for rescuing Ariok here!,) which will be the orc-heavy part of the raid. Then we'll get Bastion of Shadows with Iskar (the leader of the Arrakoa Outcasts! Uh oh!) Socrethar (leader of the Sargerai, and apparently not dead despite our efforts) and Velhari (no precedent, but based on the name I could imagine maybe Draenei/Sargerai.) Likely we'd then get Destructor's Rise with Zakuun (no precedent, probably a demon,) Xhul'horac (ditto) and Mannoroth. And then Archimonde will come through the Black Gate, which appears to be standing in the ruins of the Dark Portal. So where Kormrok, the Iron Reaver, or the elusive Gorefiend fits in is the big question.

Update: There are two achievements for the Halls of Blood and Hellbreach (also, apparently Hellbreahch is a word my autocorrect thinks exists.) Hellbreach appears to be the first wing, composed of Hellfire Assault, Iron Reaver, and Kormrok. Halls of Blood will have Hellfire War Council, Kilrogg Deadeye, and Gorefiend.

So breaking it down, it looks like Hellbreach will be where we fight the main Hellfire Citadel defenses, ending with what I assume is a Fel-Corrupted Magnaron given the name. Then it's either Bastion of Shadows or Halls of Blood. Bastion of Shadows will, I think, be the "Sargerai" wing, especially if Velhari is some form of Eredar. Halls of Blood will be the "Orc" wing finishing with us facing off against the horrifying monster Gorefiend - who actually looks more Old-God-like than demon. Maybe some Pale stuff got in there? Destructor's Rise will be the demons-proper wing, and Black Gate will be the single-boss wing with the final fight against Archimonde.