So Blizzard has said that we 'might' see Sylvanas this expansion, but i'm thinking, why the hell wouldn't we! The Undead are firm allies with the Blood Elves, who are planning a Coup d'état alongside the Darkspear Rebellion, to bring a sunset to Garrosh's totalitarian regime. If Garrosh is removed from the picture, she would have full use of the plague and Val'kyr, as well as any other in-humane acts of terror she has up her sleeve, to use to reclaim the entire kingdom of Lordearon. So why is there currently no sign of her?
The Alliance will likely be using a vast majority of there fleet in the Siege of Orgrimmar, so even if she doesn't make an appearance in the Siege, she should have lore focused around her second attack on Gilneas. With the Stormwind navy occupied, and the 7th legion busy laying waste to Orgrimmar, the only thing left in her path is the Worgen. With Gilneas defeated, the only Alliance force left in Lordaeron would be to the East. The Hinterlands and Arathi, both of which the Undead have settlements in. (Small, but could be upgraded in the future)
Gilneas, Tirisfal, Silverpine, Hillsbrad and Western Plaguelands. All, would be under her command. Dark Lady watch over you.