Yes, but everyone knew of Illidan from the beginning, he was the planned ultimate boss for BC. We don't know who the possible antagonists are, but we do know of the watchers (i.e. Freya, Thorim, etc.) so it would make sense to identify an encounter by a term we can all recognize and relate to.

I heard that it's heavily implied that Archimonde is behind the corruption?

Hard to believe, but if so... I'm starting to think that the chips are piling up towards the Emerald Dream being the next expansion subject. If the comic is covering it, and we've got the Stormrage book coming out... it seems like they're fleshing out the ED lore somewhat, eh?

I'd most certainly be interested in an Emerald Dream expansion. Would give them an excuse to revisit a version of Azeroth classic, including finally unlocking Mount Hyjal. Though I do worry if there would be enough content in an expansion solely about the dream, who would we fight? Besides the mysterious big bad that none of us know about, that's not exactly going to excite the masses.

Archimonde corrupting the dream was originally a theory people threw out there because his remains(Skeleton) is on the original World Tree Nordrassil. People theorized that because he died on the tree, and the tree had such a powerful tie to nature that perhaps he gained access to the Emerald Dream to escape death in a corporeal form, this is partially supported by the revelation that Cenarius is still alive in some form in the dream while his mortal shell died to Grom Hellscream. Keep in mind any speculation about what's causing corruption in the Emerald Dream at the moment is just that, speculation, there's been no officially released in-game hint or novel hint or even RPG book hint about specifically what's corrupting the Dream, and the evil is described as ancient. As powerful as Archimonde is, it doesn't really fit with his style if that makes sense?

A few things we know about the dream currently:

Nightmares are running rampant, these include and are not limited to actual manifestations of people's greatest fears. In the upcoming novel Stormrage, Metzen hinted that we'd get to experience one of King Magni's nightmares as it shows itself in the dream.

While on the subject of Malfurion, he has been in the dream since a few months after the Battle of Mount Hyjal, his body is kept hidden in Moonglade and protected(Not visitable in-game).

The corruption is very strong and the green flight is suffering greatly. 5 notable members of the Green Dragonflight have been corrupted(Emeriss, Lethon, Eranikus, Ysondre, Taerar), 1 was saved/cured after a great battle @ Moonglade(Eranikus). While Ysera's condition isn't completely revealed, it has been hinted that the dream is taking her toll on her via the quest Cycle of Life - Quest - World of Warcraft, confusion for one in her position(An Aspect) is never a good thing.

Druids can be corrupted as shown by the corruption to the Druids in the Wailing Caverns. Many Druids fear traversing the dream at this moment for this reason alone at the moment.

The Nightmare is attempting to use the Dream to break through into our world.

In addition to what's been said above, what happens in the Emerald Dream strongly effects Azeroth and vise-versa. IE corruption or destruction of nature can be felt in the dream by Green Dragons/Druids. Corruption within the dream can corrupt areas inhabited by nature such as the Great Trees in Feralas, Hinterlands, Duskwood, Ashenvale as well as the areas around it and it's surrounding inhabitants, Teldrassil is another growing example of this.

I re-stumbled upon this a few days a go and I believe it was posted here a while a go couldn't find it though. The Next Expansion's Details.. The Depths of the Dark Tide - Unofficial World of Warcraft Forums

It contains some of the things that will be included in the next expansion and it says that we are going to visit Emerald Dream, but it isn't going to be a whole separate continent, but a sub-area. It might be fake and pretty much everyone is calling it bullcrap on worldofwar, but it has me a little biased.

There are nonetheless interesting concepts in it.

The leveling system's description makes me think that Blizzard is already half way there.
Basically, imagine you couldn't enter Zul Drak if you hadn't completed the Grizzly Hills Drakkuru quest line, or that Icecrown would be a "no go zone" until you saw the Wrathgate event.
Technically speaking, I guess this means using the phasing technology on zones, rather than on sub-zones.

I kind of like the idea, the concept is interesting.

The emerald dream corruption is referenced once or twice as being from an Old God, if you go back 10-20 pages we had this discussion recently whether it meant Old God as in the race, an old God like Hakkar, whether Hakkar was an Old God or avatar of one, etc... Sargeras, Archimonde, Deathwing etc... seems more far fetched.

For 3.2 people are forgetting we've not had a troll raid this expansion. Temple City of Zul'Drak anyone? There is still that massive main building we don't even touch through the quest line/instance.

Let's not forget that there is also the mystic raid that was briefly talked upon which would have been brought in if WotlK would have been delayed. ("We have another raid instance prepared should there be any delay on the WotlK release"). Wheter this one is back on the bench or just rolled into regular schedule I don't know but if the latter then it shouldn't be tied too much to Northrend per se as it would have had to be available in regular TBC.

It would be nice if they could turn "The Temple of Gun'Drak" into a 5man simmilar to Magister's Terrace in pacement, it being a step up from normal 5man instances.

I personally suspect that we will be fighting the watchers one way or the other. They probably don't do this out of free will; while admittedly the main antagonist of the Kalecgos encounter is not Kalecgos himself (who is being corrupted); we have never seen an encounter's name referring to an entity which is not hostile to us in that encounter. My personal suspicions are that we will somehow have to redeem them and that they will play a role later in the instance. Considering the supposed power level of the Old Gods, and the upcoming patches likely having to knock down Arthas a bit before taking him on head on, I can't imagine them having us fight a full power Old God and win on our own. Since I'd find it a rather lazy and lame thing to again have us fight something in a weakened state ("Too soon, Executus, you have awakened me too soon!" and C'thun come to mind), and for me the full power option is also out (also considering Deathwing and other Old Gods in future expansions), the most likely angle I'd see for this would be to have powerful assistance. This could be in the form of taking part in the encounter (be it hopefully a bit more effective than Maeiv, thinking more in the direction of the Kil'jaeden fight), or in terms of a buff (something like Essence of the Red). Whichever of the options would be in, I don't see a better option for this kind of 'assistance' than the redeemed watchers which were originally appointed to watch over Ulduar.

Some other assorted thoughts about things which have been previously mentioned:

-While the current preview of Ulduar points to YS being the final boss, this is quite different from the vibe I got from the recent interview with Metzen for Blizzcast. To me that suggested that the whole Old God corruption from YS is part of a greater plan (see the getting all the dragonflights somehow occupied), and that this story arc might be pushed beyond the scope of WotLK even. (Which wouldn't be surprising with things like the Emerald Nightmare and Azshara still around) On the other hand this might have been more about the Old Gods as a whole, with YS being the one to fall this expansion. (And makes the 'full power' option to fight YS even less likely, given the future opponents we might get)

-I do not hope that we end up with another troll instance, because by now it would be a bit stale. More importantly though, I cannot see any way in which it ties into the main Lich King story arc, as I do think in 3.2 something needs to happen which starts to create a situation in which Arthas becomes a somewhat viable target. (As opposed to Matthias Lehner's current view on our efforts)

-I think in the interview I referenced above as well, Metzen all but confirmed that the masterminds behind the corruption of the Emerald Dream are the Old Gods. Who their agents are in this, if any, remains to be seen I guess.

-Some people might take the idea that YS is all under Northrend a bit too much in physical terms. To me this seems like this is more of a description that his corruption spans the entire continent, through the whispers and corruption which he has been using for a good long while. (probably steadily expanding his realm of influence) To me 'tendrils' and influence are more a figurative description of his effect on Northrend, rather than stating that his physical body spans Northrend.

It would make sense for Arthas to show up. Arthas has been grooming us for something big and does not show much respect for all things Old God. And he has become heavily intertwined with Yogg-Saron and the Faceless Ones. I could very much see a "THIS WORLD IS MINE!" moment.

That doesn't mean Arthas would necessarily know what he was doing, that it would work, or that he could even actually get his hands on Yogg-Saron. The Lich King seems to fall back on Arthas' personality for combat - walk up to it and hit it with Frostmourne. That's probably not the best way to go about fighting Yogg-Saron.

Something to ponder about Ulduar;

If the Titans "would not / could not" destroy the 'old god of death' locked in the city, what does this say about our (the players) ability to kill it? I mean, you can interpret the intro lore a few ways.
It could be that the Titans didn't have the power to outright destroy the thing so they just locked it away (doubtful.. i'm sure they could nuke the planet if they wanted to). However, based on what was already revealed about the curse of flesh, it seems more like the Titans couldn't kill this old god due to it having some kind of tie to Azeroth. I don't have the entire conversation available this second but in Halls of Stone, during the Bran ring event, it is revealed that the Titans couldn't kill the old gods. Doing so would apparently "kill" the planet due to the curse of flesh.

This of course really begs the question; what do you ultimately do in Ulduar? It just seems somehow hokey that the Titans locked up some powerful entity due to not being able to exterminate it outright, and yet the players are going to storm in and do it themselves? I also have to think that the Lich King has his hands in this somehow as well, but to what ends obviously at this point is speculation.