Here are two ideas of what expansion will there be after Warlords. To my mind, both seem possible, though, as with Warlords, Blizz are common to give out something that no one could predict. So, choose whichever you think is more likely to actually be next, or the one you like more, or for none if you dislike both ideas or think there'll be something completely different.
The Dark Below.
Of cause, it's not likely to be the actual name, but clearly shows the theme. And I'm sure that South Seas AND Emerald Dream will go together. As Metzen explained, Dream will work much as elemental plane in Cata, making them zones parallel to normal world zones. So, it was quite obvious that neither dream alone nor seas are not sufficient to be an expansion. But them going together would be a drastically epic story to tell.
Preliminary: it structure it reminds of both Cata and WotLK. Just as with elemental unrest and earthquakes, 2 things begin to happen -
1) People all over Azeroth begin to complain at nightmares at night, some time later even start to show agressive behavior at lunatism, depressions at day.
2) Storms, tsunami, fogs and sudden weather changes, like hard rains or typhoons in previously calm zones. Sea growing more dangerous to sailing.
All that becoming heavier in frequency, until the...
Event: multiple things happen at once. People are in panic as all this looks really apocalyptic.
1) Skies over ALL Azeroth are covered in thick clouds, making it almost as dark as at night, rain begins to flow.
2) Earth trembles, as from the Maelstorm Ny'alotha rises, opening the Rift of Aln. Rocks turn alive, crushing houses, ground itself turned against us.
3) Sea is RAGING, drowning all ships in fierce storms, washing away all coastal towns with tsunami.
4) Portals to the Emerald Nightmare open up, all living creatures near driven mad, while deformed, terrible beings spawned from that realm attack us.
5) From the sea turned open, an endless army of nagas, faceless ones, zandalari and sea monsters never seem before march at our main cities.
6) Periodic events for all the expansion - much like DW surprise attacks. One being all sky covered in clouds, rain begins, lightnings randomly strike and can kill you, typhoons also lift you in the air and kill - this is Azshara casting magic at the zone. Another is smaller armies of naga\faceless\zandalari make an attack. Third is nightmare event, when at night npc turn hostile and attack, mobs and nightmare beasts attack too.
New zones -
1. Emerald Dream - on continents and in the centre. Central called Rift of Aln\Black Forest, a max level zone.
2. Kul'Tiras (Alliance).
3. Tel Abim (Horde?).
4. Plunder Isle - pvp battleground.
5. Zandalar.
6. Echo isles.
7. Broken isles.
8. Underwater zones.
9. Yet unknown territories in the sea.
Main enemies -
1. Azshara and her underwater army, naga, mur'gul, kvaldir, kraken, other beasts we don't even know about. Nazjatar being the final raid of expansion.
2. N'Zoth, his faceless ones, nightmare beasts, druids and dragons of nightmare, satyrs. Ny'alotha a penultimate raid.
3. Elemental lords - Neptulon subjugated by nagas, Therazane manipulated by N'zoth somehow to ruin our cities so they can kill us easily.
4. Zandalar trolls, under mad prophet Zul, who is desperate and made pact with Azshara, now looking to search Tomb of Sargeras and succeed where Gul'dan failed, to have a power to make his people worthy of the new age coming and rebuild troll empire again. Tomb of Sargeras being second raid tier.
Major characters to participate:
1. Malfurion. He opposed Azshara in the past, so she sees him as main antagonist. So, he's Tirion\Thrall of the expansion, who'll bring Azshara down.
2. Tyrande - surely will help him through. And perhaps killed finally?
3. Cenarius, Hamuul - Mostly dealing with nightmare issues, seeking a way to cure people from turning to each other and being possessed.
4. Ysera - after having lost her Aspect position over the Dream, she allowed N'zoth perfect chance for preparation. Now, she will accompany us through Black forest and Ny'alotha, so that we don't get overcome with nighmare upon merely entering there, possibly sacrificing herself in battle against N'zoth.
Cenarius, Hamuul and Malfurion will likely also help us through, like Keepers of Ulduar against Yogg.
So, overall this means, the expansion is mostly NElf-centric, being reminiscent of War of the Ancients times and stuff.
Now, to another one.
Return of the Shadow
By Shadow here I mean actually 2 shadows - shadow of legion and shadow of the dead. I think we all remember that LK is left inactive and that he'll one day come back. "Power was given into wrong hands", said the chinese leak, so might it be a reference to it? But at the same time, does anybody think that
there can be just another LK\Scourge-themed expansion? That would be ridiculous, of cause.
So, it'll go along with something else. And if it's a Legion's return, it would satisfy those who were waiting for legion xpac to come. Wrathion foresaw legion's return, eradicating all in its wake. But there must be someone to help it to get to Azeroth, someone powerful enough. And if we put this together....Who summoned legion the last time?
And moreover, as Blizz said, they are trying to be close to wacraft now. And that is highly reminiscent of WC3, with scourge and legion striking again, just like WoD is reminiscent of WC2.
So...
Plot: The huge war would yet again start with just 2 persons - Sylvanas and Kel'Thuzad. If Sylvanas spoils relations with horde and tries to attack alliance, she'll most likely be defeated and banished. Then, the former ruler of Undercity will travel to Northrend and meet the second participant of the story - the Lich.
As we remember, his phylactery was never found again, so after some years he had a fine chance to return, though, went into hiding. Though, Sylvanas is not friendly neither to Kel'Thuzad nor to Lich King, she is eager for vengeance (another Garrosh, uh?) and makes a pact.
Together, the two release LK from ice, maybe killing Bolvar or wiping out his benevolence, idk... Perhaps, they eradicate Bolvar's spirit to let evil Ner'Zhul repossess the Helm again. So, Scourge is active again. Lich pillages Nexus and takes Focusing Iris.
Either he begins to channel a spell at Nexus, or, ironically, again Dalaran will be ruined and used by Kel'Thuzad? Either way, Kil'Jaeden is summoned back, a pact made between the Legion and the Scourge.
This time, the enemy is even more dangerous than during the Third war - KilJaeden is more cunning and powerful that Archimonde, and Scourge is now lead by ACTIVE lich king, though he's probably weaker than Arthas was, but still almost unbeatable.
What chance do we even have?
Main enemies:
1. Kil'Jaeden - finally in his full powers, as epic as Hyjal events. Final boss.
2. Lich King II - maybe even in the same raid as KJ - not just 1, but 2 end bosses?
3. Sylvanas - many have dreamt to loot her since she became delving deeper and deeper into dark experiments, turning Forsaken into another Scourge.
4. Kel'thuzad - well, maybe 3 times is enough?
5. Some suspect, that we shouldn't be surprised of anything. Who knows who may defect to join such a force out of fear, vengeance or power hunger?
Main characters to take part -
1. Velen - yes, that's what he warned about, and this time his arch-nemesis is here! An epic duel is inevitable....
2. Wrathion - he's a hero of the day, too - now it's time to see if he did his job well to prepare Azeroth for war. And to see for what is he ready to save the world. He foresaw "burned cities, slaughtered citizens, all in fire" - here it's time to fight back!
3. Jaina - since she's in Dalaran, in Northrend, she'll be the first to encounter the old new foe. What awaits her?
4. Ner'Zhul - if he'll end up defecting at our side, he'll certainly be horrified to know what crimes he commited in our timeline. Then, becoming one of the Horde's spiritual leaders, he is desperate to fix the harm he made to his new home and even help to fight himself in an evil lich form. Quote: "Q. Will Ner'zhul appear at any point in 3.3 or the near future?
A. Well, he is a chunk of the Lich King now. But if you mean will he appear as an orc, we're not ready to tell his story just yet. We have a lot of stories left to tell, but his is a good one".
5. Anduin, Shaohao, Cenarius, Vol'Jin and others - all those guys are much-needed leaders of their people, they are to stand against panic and not allow people despair, unite both factions against the most terrible force the world yet stood against.
New zones - all that comes to my mind is Northrend being remade, now half-molten by the fires of the Legion, and a portal to the Twisting Nether through Nexus is opened, but I guess most part of the action will take place here on Azeroth.
So, the Reign of Chaos is about to repeat itself, even more dangerous and terrifying than the last time. And this time, we, mortals, don't have Nordrassil or ancient spirits to sacrifice to save us. It is our, mortal burden, to stand against a foe which seemingly cannot be defeated.
Are you prepared?