If you pay attention while playing, you can spot at what seems to be hints of future content.

When Kael is killed, and the Verdant Sphere turned inm this message plays

-Silence descends upon Shattrath
-A'dal's thoughts invade your mind
-Kael'thas Sunstrider has been defeated by $N and (his/her) allies
-The time to strike at the remaining blood elves of Tempest Keep is now.
-Take arms and let A'dal's song of battle empower you!
-Kael'thas Sunstrider yells: Your monkeys failed to finish the job, naaru! Beaten but alive... The same mistake was not made when we took command of your vessel.
-Kael'thas Sunstrider yells: All for what? Trinkets? You are too late. The preparations have already begun. Soon the master will make his return.
-Kael'thas Sunstrider yells: And there is nothing you or that fool, Illidan, can do to stop me! You have both served me in your own right - unwittingly.

Scryer NPC in Netherstorm, Magister Theledorn, says Kael has entered into an Alliance with the Burning Legion, turned his back on Illidan etc.

"As you may have noticed, there are servants of the Burning Legion all over Manaforge Ara. Kael'thas has entered an alliance with the Burning Legion. He must've caught wind of Illidan's descent into insanity and switched paymasters accordingly. "

Is this Sargeras they're referring to? Shrug, this could be unrelated, and "the master" is a rather common term of fealty, but...

While doing Netherwing quests in Shadowmoon, you may notice a giant Black Dragon land in Dragonmaw, and begin to speak with Overlord Mog'hor, her name is Lady Sinestra.

-Lady Sinestra says: Overlord Mor'ghor, I presume... A pleasure to finally make your acquaintance. I am Lady Sinestra.
-Overlord Mor'ghor says: I will not drag this out any further than it needs, -Lady Sinestra. You have bent my ear, now tell me what it is that you want from the Dragonmaw.
-Lady Sinestra says: You have no doubt heard about Nefarian's failures on Azeroth... While he has fallen, the experiment continues. My master... He continues the work that his progeny began.
-Overlord Mor'ghor says: The... The master? He lives?
-Lady Sinestra nods.
-Lady Sinestra says: You were once a chief lieutenant, Mor'ghor. Your work in Grim Batol is not easily forgotten. Now... We need the eggs that you recovered. The Netherwing eggs. They are, after all, a product of the master. We will pay whatever price that you ask.
-Overlord Mor'ghor stammers.
-Overlord Mor'ghor says: Ye... Yes, yes... Of course. We need only the crystals and ore from this place. There is... We will need mounts.
-Lady Sinestra says: The master will be most pleased with this news. The Black Dragonflight will provide you all that you ask. You will be allowed to ride upon the backs of our drakes as needed. I thank you for your graciousness, Mor'ghor. I must now take my leave.

Now, she makes the same reference to her superior as "the master."They both share a name, Deathwing the Destroyer and Sargeras the Destroyer, although likely just coincidence. Am I just bored at work and seeing mountains in the place of molehills, shrug, I think we will be seeing more of these two in the next round of content.

Yeah the first definitely sounds like it's referring to Kil'Jaedan which would be one hell of a fight

Well, in terms of Legion lineup, Kil'Jadean is below Sargeras, although it would likely be we would fight Kil'Jadean, which I didn;t even think of. The second is obviously Deathwing, although the intertwinement of the two is unknown, although could be possible.

While Alexstrasza may be keeping the Demon Soul under lock in Grim Batol, it is very unlikely now she is also holding Neltharion, as he is obviously back "in the game". No pun.

The article that mentioned guild banks also talked about future TBC content. Kil'Jaedan was mentioned as a boss for the next big raid dungeon.

It also mentioned that hero classes were dead and would probably never be implemented.

Sargeras is dead for all intents and purposes. I don't personally think we'll <ever> fight him.

AFAIK, Neltharion is the one who has (or at least very recently had) Alexstrasza imprisoned - he used her to mother the first dragons of the Black Dragonflight. The Demon Soul, is, I believe, still held by Neltharion too.

Edit: Scratch that, The Demon Soul was destroyed by Rhonin.

We said the same of Archimonde, yes? I don't think we can rule anything out

Would you happen to have a link to this article? I read one with a bit of info on guild banks, VOIP, and designing Zul'Aman as a 10-man, but lacking those other bits.

Grim Batol ?

Oh dear lord, I hope not.

Yea, gonna need a source for this. I'd say it's too soon to fight someone as powerful as Kil'Jaeden, but given the fact that you fight Archimonde.. I don't know. It would seem pretty silly if you managed to effectively dismember the Burning Legion, the great evil force of the universe.

However, I agree with the assessment that Kael is referring to Kil'Jaeden as his master and that Deathwing is the other master, although the Black Dragonflight and Burning Legion are probably allied.... but I can see a Deathwing instance being added before a Kil'Jaeden one.

Dead? Nah... he's been banished to the Nether or something, but I can see his revival being a plot point in a future expansion, namely the Maelstrom one whenever that comes out. Will we fight him? Maybe in a nascent form, like C'Thun..

The red dragonflight stronghold in Wetlands... it's on Blizzard's agenda, I guess.

It's been more or less confirmed that ZA will be 10-man, which is fine by me, since I think a casual UBRS-like 10-man is what's lacking in the game atm. However, ZA seems to be a little big to fill that role, so... who knows. In any case, it's not going to be a Tier 7 instance or whatever, I'm sure it'll be less than Tier 5.

But as for Blizzard's actual agenda... it seems like Zul'Aman is the main thing being worked on, and after that... who knows. I'm still looking forward to the Sunwell Plateau being added.