Title of the expansion is as listed above.
The basic premise:
Iridikron is helping Xal'atath to awaken Azeroth, which will in turn bait the Titans to return.
We will indeed be heading for Avaloren, located north-west of Kalimdor. You can actually already see the continent of Avaloren now on the globe featured in the new Hearthstone TITANS expansion game board. This was an intentional easter egg. There is an additional landmass here which I do not know about.
There will be a third faction introduced, although not in the same vein as the Alliance and Horde. You won't be able to select this faction on the character creation screen, but will be able to defect to them during the expansion if you choose to. They are called the Tidesworn, they are neutral and are primarily made of of privateers and mercenaries. Pirates, basically. I do not know the benefits of joining the Tidesworn, but I assume it will involve alternatives story arcs, cosmetics, mounts etc.
The Tidesworn are headquartered at Gadgetzan, which is now vastly bigger, resembling it's depiction in Hearthstone and the Traveler book series. This will act as your major city hub should you choose to work with this faction.
Avaloren is home to Uldaz, the final titan facility. Yes, there is an Old God here, but this one is not like the others. This Old God arrived on Azeroth before the four we know so far. It was the "original scout" sent by the void. It was imprisoned long before the four Old Gods arrived and formed the Black Empire. Iridikron wishes to use this creature as a weapon against the Titans on their arrival, but I do not know the details of this plan. I believe this will be the focal point of the first raid.
Avaloren has five zones. Khaz Algar and Uldaz were among them. There was mention of a place called Ebonwood too but I'm not sure if this is an entire zone or a sub-zone.
There is a heavy Ethereal presence on Avaloren, they are sided with Xal'atath. There has also been significant customization development on these models which would suggest a potential playable race.
There is also a heavy Naga presence on Avaloren, they are sided with Azshara. There appears to be some competition between Xal'atath and Azshara to become something known as the "Twilight Sovereign", this has something to do with the void's throne of power. We will get caught in the middle of this conflict.
That's all I have for now. If I receive any other details I will post below.
EDIT: forgot to mention. Old world is getting facelift. Gradual rollout of modern assets including buildings, trees etc.
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To clarify, if you join the Tidesworn you do not leave your current faction. But there is extensive group content specifically for Tidesworn players.
UPDATE: I want to clear some things up.
1. No, there are no new classes being introduced.
2. AFAIK there are no new races being introduced, at least on launch. Blizzard are however focusing more on existing race customization.
3. The controversy surrounding the final titan facility, jailing of the original old god and the first arrival of the titans will be explored during the levelling campaign. Basically, the titans lied to us about the ordering of azeroth. This has already been established in retail. There is an entire chapter they left out. (Blizz retconning again)
4. It is my own personal belief that the Ethereal are being prepared as a playable race. I have no official knowledge of this. Naga will NOT be playable.
5. The third faction is more narrative than anything. It will not have any impact on battlegrounds or PvP in general. As I mentioned above (which people generally ignored) it is NOT faction in the same sense as the Alliance & Horde.
6. Do not worry if this sounds lackluster. This is, by far, the largest expansion Blizzard have ever released. The information I have been provided is barely the tip of the iceberg.
7. Avaloren itself is MUCH larger than the five zones we are visiting in this expansion. The rest will be inaccessible (like the Ogre continent in WoD)
8. Turalyon will be a pivotal character during the expansion.