So, lets do this, will likely get buried among the others, but hoping some good ol' digital archaeology will pop up after the fact and validate this here leak.
Most of the other fan fiction are easily identified because its information they would not have unless they had a source in each department. I usually hear further in advance than this, but they are being Legion secretive this year. I've heard from 2 main sources, one does infosec for an online company who have an article ready to release at the drop of a hat, the guy mainly scrapes sites for vulnerabilities, magecart breaches etc these days, but does work for a few of these publications and gets to vet these pages before they are pushed public at the moment the NDA expires. My actual Blizz source is a 3D artist, specifically a prop designer.
A lot of stuff here I'm making educated guesses from what each has told me, Ill point out clearly when I working on limited info, but its not like I've seen the intro cinematic or any promo material, and I have limited access to the lore reasons behind much of this.
I don't know the title, "The Icy Winds of Northrend" was considered, as was "Ascendant", amusingly apparently an early area was dubbed Shadowkeep, but that was changed due to Destiny's latest expansion, possibly as a last parting jab by Bungie, but I digress.
Entire expansion takes place in Northrend, sorry, no other revamps, at launch at least. However, its not entirely Shadowlands and Death themed as many have partially correctly guessed.
Opening cinematic must be some kind of confrontation between LK Bolvar and Sylvanas, she is almost killed, but successfully shatters the frozen throne during the battle, because the pinnacle of Icecrown becomes a huge tear in the sky, not unlike Argus in the sky during Legion, leading to the Shadowlands themselves. The precipice of the Shadowlands is reportedly the largest zone ever introduced to Warcraft, but is still only a single zone, which is dominated by a giant dark citadel (this is the formerly titled Shadowkeep) which bars the still living players from entering the true heart of the realm of death. This will likely be our first raid and introduction to who some have correctly guessed as the true incarnate of death, Mureh'zala, but I don't have a certain on that.
The other main theme of the expansion, which has been kept very quiet, is further south in the VASTLY changed Dragonblight zone, which has been renamed, to what I don't know. Wyrmrest has been mostly excavated from the surrounding Ice, and has been greatly expanded, with the interior several times its original size.
The five flights Sanctums have been turned into zones in their own right, still accessed by the old portals in the base (now central tier) of the citadel. These sanctums will serve as the order hall equivalent for players, except instead of an order hall for each class, each SPEC will be pledged to one of the five flights. I'm not entirely sure how they will be divided up, but the example I was given was Mages will pledge to the Blue flight as frost, With Jaina and Kalec as key parts of the introductory scenario for that Spec, I'm guessing this is the one they plan to demo at Blizzcon, as this is one of only 2 which are finished currently. Though it wasn't confirmed, Id guess Fire and Arcane mages get assigned to the Red (maybe Black, will they reserve the Red flight for healers?) and Bronze flights specifically. Each sanctum will have a "trial" unlocked slowly throughout the first half of the expansion which will serve as the challenging single player content, a la mage tower, for this Xpac.
Speaking of specs, again, I don't know this 100%, but it seems like every class bar druids could be gaining another spec. There is reportedly a keystone artwork featuring a Demon Hunter clad in what looks like the BE heritage armour wielding the legendary bow Thoridal, so we may be getting a second bow using spec, but after I got excited I was told this doesn't mean a second round of artefact weapons.
No new races at launch, just more DK options as everyone now knows anyway.
The big one, I've heard a lot of people are expecting tinkers, but apparently the new class is Dragonsworn, a class based on channelling the essences of the five flights, mail wearer, has a transformation mechanic, and will be only class to have FIVE specs.
On the subject of specs, apparently they are walking back the design of them being entirely specialised, to more of a BC/Wrath era design where a spec augments and alters some core abilities and aspects of your class rather than it being almost entirely different for each spec, there will be a lot more shared class abilities between specs than previously.
Two main city hubs, both neutral, Wyrmrest and the forward base in the precipice of the Shadowlands (its possible that this "forward base" is Naxx, since its has been removed from its original spot), perhaps a sign the conflict between A+H is truly done, and that the LK is on side?, but no real confirmation of that either. Interesting Ulduar is apparently getting a huge overhaul and will be included within the updated Northrend instance, and has many of the assets associated with player hubs, but no info beyond that.
For some reason the Argent tournament grounds are still there and have new side content.
Azjol Nerub is likely not a new zone at launch, but there are new, HUGE environments being developed for down there, I hope its finally a zone and not just for another raid / dungeon.
They were experimenting with re-colourable armour pieces a few months ago, no word if it came to fruition, but reportedly they had to have a small team go through and re-do the UV unwraps for a LOT of the old armour, so promising I think.