The expansion begins with opening of the Gates of Greymane Wall, a mere few weeks after Illidan's defeat at Black Temple. Suddenly, droves of refugees, pursued by extremely powerful murlocs, led by Nagas, enter Silverpine Forest. Agents of Undercity and Dalaran are sent to investigate. What they discover is a startling revelation...
Illidan survived Burning Crusade, and fled Outland. Kael'thas defeated, his demonic and orc armies scattered, Kil'jaeden majorly pissed off... what possibly can he do? He's got only one ally left: the Naga.
He obviously has already learned from the Naga all about the mysteries of the deep. The murmuring voices from the abyss, the powerful entities (entity?) that is willing to share its force... so he goes to the Maelstrom and allies with Lady Azshara herself - an old friend, by the way. The murloc armies are at his disposal. The power of Kraken may be his, and Neptulon is approached by his ambassadors, asking for alliance - he must be stopped, of course. Here you'll have a chance for yet another troll dungeon, for those who like troll dungeons - or maybe a revisit to Zul'Gurub? New wing for example? (Caverns of Time wing showing Zul'Gurub before the downfall?) After all, the Gurubashi have had something very suspicious to say about Neptulon...
Kingdom of Gilneas falls first under the attack of underwater army. King Greymane dies, and the gates of his Wall open to reveal a ruined nation full of dangers. Adventurers venturing to Gilneas discover documents that tie some of the recent events to activity of Lord Prestor - Deathwing himself. They are sent to Grim Batol and to red dragon hatcheries at Crestfall to investigate further. Area east of Grim Batol is now opened for travellers, although very dangerous, as it's probably mostly inhabited by fighting black and red dragons...and some Wildhammer and Amani troll outposts. Kul'Tiras, of course, becomes active again... Jaina Proudmoore plays a new unexpected part... and the Missing Diplomat quest continues to a surprising conclusion! What does Kul'Tiras have to do with all this, and why was the king missing all this time? (and who's actually being held in Tol Barad prison?)
Horde-side, Darkspear Trolls return to their homeland to investigate the true reasons behind murloc infestation that caused them to leave in the first place. And of course, you get to visit Zandalar. Perhaps a land west of Stonetalon Mountains is opened, where the Sea Giants dwell and have some interesting stories to tell?
Makrura request to join one of the mortal sides as they are being slowly wiped out by the Naga... And the benevolent Sea Giants offer their help in the new war, as they feel the need to fulfil the duty given to them by the Titans, and fight their corrupted brethren.
The Old God is "found" beneath the Maelstrom (where does all that water go, anyway?), and to find truth about him, you have to venture to a new instance west of Tirisfal Glades.
New instances: 70+ Zul'Dare, 72+ Tol Barad prison, 73+ new Troll instance, 74+ Kezan instance, 76+, Grim Batol, 78+ Tomb of Sargeras, 80+ Old God's chaining place beneath Tirisfal Glades, 80+ Nazjatar and Neptulon's Lair
New regions: 70-72 Gilneas (with Zul'Dare), 71-73 Plunder Isle and Darkspear Islands, 72-74 Kul Tiras (with Tol Barad), 73-75 Zandalar, 74-76 Undermine, 75-77 Eastern Wetlands (with Grim Batol), 76-78 Broken Isles (with Sargeras Tomb) 78-80 Maelstrom Eye (with Nazjatar),
Alliance: Dalaran, Theramore, Wildhammer Explorers, Azeroth Navy
Horde: Sylvanas Rangers, Zuldazar, Amani Scavengers, Edj Trolls (Kezan)
Neutral: Makrura, Sea Giants, Kul Tiras, Red Dragonflight, Undermine/Bloodsails (like Aldor/Scryers) Edit from Thermiel, Bloodsail is allready a faction... I'm friendly
Place in future storyline: you need to control the seas if you want to invade Northrend. Just ask the Scarlet Crusade. This may be the last attempt on Illidan's side to cause chaos, and once he's defeated here, the only possible next step is Arthas. Perhaps Illidan will still play a role in Northrend events, but no longer as a powerful enemy.