In a recent video posted by Taliesin & Evitel Do Games we learnt that both The Mechagon Megadungeon (WoW's new dungeon) and Azshara’s Eternal Palace (the new raid) will open at the same time when patch 8.2 arrives. The couple were invited at Blizz's headquarters to play-test 8.2 on the PTR and also had a discussion with game director Ion Hazzikostas and the whole encounter team behind the new content. You can watch the full video below and it contains some interesting info:

We've Played 8.2! Here's Tons More New Info Not In Blizzard's Presentation...

From wowhead:

• Mechagon Megadungeon

In the depths of Mechagon, is located the Megadungeon!

• A mega-dungeon, similar to Karazhan in size, unlocking alongside Azshara's Palace a few weeks after the start of Patch 8.2 in order for players to unlock their new Heart of Azeroth bonuses and explore the new zones.
• King Mechagon wants to turn anything flesh into robotic parts.
• Mechagon will launch as Mythic-only mega-dungeon, and be split into two smaller Heroic and Mythic+ difficulty enabled dungeons in 8.3.
• Eight bosses, with the devious King Mechagon as the final boss.

  • King Gobbamak: Trogg, a robot, defensive tower)
  • Gunker
  • Trixie & Naemov
  • H K-8 Aerial Oppression Unit
  • Tussie Tonks (a robot + a machine)
  • KU-JO (a mechanized dog)
  • Machinist's Keep
  • King Mechagon

• Azshara's Eternal Palace

Deep within Nazjatar, we will find the queen of tides' abode, Azshara's Eternal Palace.

• The raid will launch alongside Mechagon a few weeks after the start of Patch 8.2 in order for players to unlock their new Heart of Azeroth bonuses and explore the new zones.
• Armor and weapons will be themed after Nazjatar and the Naga.
• Confirmed fully underwater boss in the raid, which does give advantage to druids due to Aquatic form. It will be a stationary boss with players moving platform to platform; ground effects will always hit the boss, though there are some 3d requirements in the encounter (air bubbles, moving vertically). Some swim speed bonuses such as the fishing artifact will be disabled, but everyone will have infinite breathing.
• 8 total bosses

  • Naga-Centaur
  • Rage of Azshara
  • Underwater Monstrosity
  • Lady Precillia Ashvane (now a horrible water monster!)
  • The Hatchery
  • Naga Council
  • Herald of N'Zoth
  • Queen Azshara

There's not release date for 8.2 as of yet.