Maybe an Area where almost all the living there became Undead Forsaken.
Gilneas/Lorderon area quest where you have to rescue survivors.
If Gineas, you turn Survivors into Worgen and fight off the Undead snd take back Kul Tiras, but when they hear that the change was permanant, thay seal themselves off from the rest of the world like the Gilneans.
The Warcraft RPG book Shadows & Light might hold a clue as one of its short stories is about a group of adventures that had plans to build a device to raise Nazjatar to the surface of the ocean. It raises the question as to whether the palace could be brought to the surface. If they managed, it may have become the island in the concept art.
In a remote kingdom, the battle between the orcs and humans took place atop a huge creature. Created from the eldritch energies unleashed by Medivh during the birth of the Dark Portal, a giant, carnivorous beetle roamed the lands of Azeroth. Destroying ancient forests to satisfy its voracious appetite and crushing all that stood in its path, the beetle became a menace to both the noble armies of the seven kingdoms and the savage green-skinned invaders. In an awesome display of sorcery, the likes of which have not been seen to this day, the great creature was polymorphed by the mages of the Violet Citadel just as the orcish death knights unleashed a massive barrage of Decay spells. The result was the death and transformation of the beast into what is now known as Beetle Island.
Use a mount to chase the beetle or fight Medivh with Sargareus inside him.