In this expansion concept, Azshara is the main antagonist (or, at least, a pawn that drives the story forward - like Sylvanas). She took the Void portal in BfA, seeking for "the true throne of power".
That portal led her to an alternate ancient Kalimdor (yeah, i know how much you guys hate alternate reality expansions [Draenor], but we can't mess with the main timeline), at the height of the Night elven empire.
She wants to change something that she did (like Garrosh wanted) in the past. Either, her Burning Legion allegiance or her servitude to N'zoth. She knows something that we don't. Somewhere in that place "lies the true throne of power". Whether it is by tapping directly to the Well of Eternity to gain Azeroth's power, or something that has to do with Elune (who is a prominent figure there).
We will take her out the same way we did Elisande - by combining different elven factions and groups together, as one. This threat "hits close to home", as they feel that they are somewhat accountable for Azshara's actions and the Sundering.
Therefore, the potential class to be added to that expansion is elven-based.
New Class: Night Warrior
This class will combine the Night Warrior (Priestess of the moon) with the Warden.
Zones
The continent will feature most, if not all of the elven settlements listed on that map.
Zin-Azshari (Elun'dris/The Eye of Elune)
This zone will be the high-level zone of the continent (much like Suramar was). We will deal there with Azshara herself (in her elven form) and the forces she amassed, in a similar manner to Elisande. This fight will be of great importance - not just for the goal of stopping Azshara, but also for the purpose of securing a homeland for the Night elves.
Eldre'Thalas (Dire Maul)
This zone will deal with the Shen'dralar Highborne. We will see their origins, culture and way of life while also dealing with those loyal to Azshara. They are experimenting in whatever magic Azshara has set her eyes on. A group that will eventually help us are those that joined the Kaldorei. This place will be another candidate for a new night elf home. The Coliseum will serve as a PvP arena.
Then'Ralore
This zone will be a lush night elven Jungle, and within it we will deal with the Emerald Nightmare. There, we will meet a group of Jungle-like, insect riding elves (a previously scrapped idea for Night elves).
Lathar'Lazal (Blackfathom Deeps)
This zone will be centered around a Night elf temple to Elune. We will learn more about her in terms of how she came to be, her relationship with the Night elves, and her powers. It will be located in the Night elven city of Zoram.
Shandaral
This zone will be a sort of museum for Arcane relics and Artifacts. Although the conflict with the Blue Dragonflight happens 4,000 years later, i'd suggest involving them into the story to make this place more interesting. By hoarding massive amounts of Arcane relics and artifacts (some of which were stolen from the Blue Dragonflight itself), they attracted the attention of some of its members, who are waging war against the reckless sorcery of the Highborne there.
Optional
Mount Hyjal
Although it features on the map, i'm hesitant to give it a place in the expansion since it exists, as it is intended, in the main universe. However, a recent updated frame of it, from the Dark Abduction trailer, drew the attention of several users here:
If it is so, then this zone will deal with the Wild Gods and Freya. We will meet the Dark Trolls, the ancestors of the Night elves, and visit their underground network of caverns. Apparently, Queen Azshara fears the Wild Gods (although she is said to be more powerful than Mannoroth, and only weaker to Archimonde and Sargeras). Therefore, she sends waves after waves of her special task forces to deal with them. We will figure out why she fears them and why she is desperately attempting to eliminate them.
Suramar
As with Mount Hyjal, this zone already features in the main universe as it is supposed to be. The fact that it is even more recent (Legion) than Mount Hyjal (Cataclysm), suggests that it is even less likely to make an appearance. Even so, i figured that i can't leave this one out while most of the other zones make an appearance.
This zone will not include the parts of Suramar we know from Legion, but the less accentuated ones like Elune'eth, Boughs of Azshara, and Ga'han.
We will meet an alternate version of Broxigar, who will help us against the forces of Azshara. Instead of Ronokon, the Pitlord, and Sciallax, the Demon Lord, we will deal with fearsome creatures made by Azshara's new powers to either lure the denizens of the place to join her cause (Sciallax) or crush to death those who oppose her (Ronokon). This zone will also be the place of operations for the forces of Death, as they work on some mysterious plan they have (inspired by the Nathrezim's experiments).
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I am aware that this is way too elvish-centered of an expansion. But, i just couldn't think of anything else that would match this setting.