The spoilers refer to the reference I make to the SoO cinematic. Everything else is my design:

**SPOILERS**

In a moment of calm, Jaina encourages Varian to strike at the Horde now when they are vulnerable, but Varian chooses otherwise

**SPOILERS**

But what if....

Varian walks up and calls out the Warchief. He states that the crimes of the Horde are not Garrosh's alone and that if the Alliance cannot have justice from him, they will take it from another who has wronged them - Sylvanas. Varian accuses her of following the same path as Arthas by increasing the number of the Forsaken, and breaking the previous war treaty that banned the use of the Plague.

The Horde rises to defend her and deny the allegations, but Greymane steps forth and denounces her with unquestionable evidence. The rest of the Horde is shocked and taken aback and Varian uses the opportunity to signal Greymane.

Some of the guards who are standing in the throne room turn into Worgen and together they attack the Banshee Queen. Fighting begins to break out, and the throne degenerates into madness. As the Alliance attack, the Horde retaliate and an all out melee breaks out among the leaders and champions.

In the frenzy, Sylvanas manages to slip away and signal her Dark Riders. She calls for the release of the plague and floods Ogrimmar with it, killing the bulk of the Horde and Alliance armies at once. Jaina and Rommath manage to teleport everyone out in time.

Both factions, severely weakened from the attack, retreat to safety. The Alliance flees to Dalaran and the remaining Horde retreat to Thunder Bluff.

------------------------------------------------

With the loss of their capital, the Horde seek to rebuild. While they stay camped in Thunderbluff, they move the capital further south in Durotar to New Ogrimmar. Covering parts of the main land and several islands, the massive city sports a distinct troll flare with plenty of canals.

Jaina, seeing the parallel between Sylvanas and Arthas seeks to prevent the rise of a new Lich Lord. She moves Dalaran into Lordaeron to prepare an assault on the Undercity.

However, Sylvanas has grown bold now that her designs have been unveiled. Possessing several Blood Elf mages, she opens a portal to Dalaran and Plague-bombs it as well. The city comes crashing down near the Ghostlands, but before Sylvanas can claim it, Rommath has seized the opportunity to claim Dalaran for the Horde!

Finally, facing both the threat of Sylvanas and a stronger Horde presence in the Northern kingdoms, Varian concentrates all his efforts on the retaking and rebuilding of Gilneas.

-------------------------------------------------

Expansion features:


HORDE:
New cities:
- Dalaran
- New Ogrimmar

Lost cities:
- Undercity

New race:
- Ogres - a brighter more open minded Horde finally embraces their old allies and fully welcome them into the Horde.

New subrace:
- With the loss of the majority of their forces, the Horde welcome: Taunka and remaining troll tribe remnants into their mids - Gurubashi, Farraki, and Drakkari.


ALLIANCE:
New cities:
- Gilneas
- Blackrock Mountain - with the Dark Irons firmly established in the Alliance, the ancient mount once again falls under Dwarven control and is brought into the Alliance. The Kirin Tor, having lost their city, take up residence and begin reshaping the top portion in a massive mage tower.

New race:
- Draconian: the failed experiments of Nefarian broke free from their prisons/cages after the fall of their master. When the Kirin Tor began crafting, they were unwilling exposed to massive arcane energy. Their experimental physiology allowed them to absorb the energy vastly altering their cognitive abilities to the point of gaining sentience. (look like Saurok)

New subrace:
- Dark Iron dwarves - with the new generation of dwarven children reaching maturity, the vast majority of them have openly embraced their new Alliance and welcome the opportunities to leave their dusty mountain and join the Alliance on adventures across the world.

That's all I got:

The Dark Below could be a reference to something bad that Sylvanas is unearthing