With Al'Akir and Ragnaros dead, their great power would be released into their respective elemental planes, unless Deathwing was already preparing a host for it similar to the Bindings of the Windseeker. The Bindings that once bound the essence of Thunderaan change everything when considering the "death" of elemental beings of great power like the lords. If Ragnaros couldn't consume the essence of Thunderaan, who was merely the son of a lord, can said essences be truly destroyed at all? Even if you can kill the elemental, you may still be able to capture its power, as Ragnaros tried to do.
Why did Cho'gall visit Princess Theradras just before mortal heroes came to destroy her? If Thunderaan couldn't be truly killed, why could the daughter of the Stonemother -- or is that the point of Cho'gall's visit? What if the purpose was never to kill them at all, but simply to use them to sculpt the face of Azeroth?
The power of Theradras, daughter of Therazane, joined to that of the Windlord himself and Ragnaros' control over the molten heart of the world would give Deathwing access to power sufficient to reshape land. But in order to move the face of the oceans, you'd need the power of Neptulon. It seems likely that the mighty kraken Ozumat was originally a primordial entity similar to the Merciless Ones who either willingly or through compulsion heeded the call of the Old Gods and turned against his nominal mastery. By usurping control of Neptulon, the final piece of the puzzle was in place. Killing or controlling Therazane would have been preferable, but without that, keeping her occupied after the shattering of the World Pillar meant that Theradras' stolen power could serve in her stead.
This means that at present, the Old Gods themselves are the most likely hosts for captive Neptulon, with Queen Azshara as the likely captor. After all, aside from Deathwing himself, no other servant of the Old Gods has the magical acumen to work such a complicated spell. Furthermore, since Deathwing was occupied with multiple efforts and Neptulon is the lord of the waters, Azshara becomes even more likely as his current jailor. Neptulon would be fortunate for it to be her and not his former masters directly. Azshara doesn't strike me as any more loyal to them than she is to anyone who isn't her.
Furthermore, Azshara's original involvement with the Burning Legion stemmed from her sense of aesthetics and perfection, as well as her pride in her own power. We are about to experience some of these events first hand in the Well of Eternity heroic dungeon.
Whatever Azshara wants, Azshara gets
Azshara's potential role in all this is interesting, because while it has been hinted for years that either Neptulon or the Old Gods were responsible for the transformation of the Kaldorei queen and her Highborne subjects when the Well of Eternity imploded and ripped ancient Kalimdor apart, that doesn't mean she's grateful to them. A being as vain, intelligent, manipulative and powerful as Azshara could well resent what has become of herself and her ancient kingdom. Azshara first considered herself a fit mate for Sargeras himself, an entity of power sufficient to help bind the Old Gods. Sargeras was once the champion of the Pantheon that rules the Titans and now in his fallen state rules and guides the Burning Legion. It was Azshara's attempt to help Sargeras reach Azeroth via the Well of Eternity that the Old Gods hijacked for their own use, and in so doing, helped cause the Sundering.
It's possible to imagine that in this case, each side believes it has the upper hand. The Old Gods may well seek to use Neptulon's power to help reshape the world and weaken the Titan's prison. We know that the very act of creating and shaping Azeroth seems to have helped bind the Old Gods within it, and perhaps it is the intent of the Old Gods to reshape it enough that they can then pull themselves free, or at least escape enough to resume their ancient, endless amusements by resuming the chaos of their own conflict. But is this in Azshara's interest? It's telling to note that despite helping distract Malfurion in Darkshore, she doesn't actually aid Soggoth the Slitherer in any way, nor does she take part in the attack on Mount Hyjal. While the Naga certainly assail the Throne of the Tides, Azshara makes no appearance here, either.
It's quite possible to imaghine that as soon as Ozumat delivered Neptulon to his fate, Azshara would have no more need to help the Old Gods with their plans. After all, the Old Gods tend to be chaotic, unfathomable entities, while Azshara sees herself as perfect. Why would she want to live in a world ruled by them? With Neptulon in her grasp, she'd have access to the same kind of control over the oceans that destroyed the Gurubashi millennia ago, once she broke his will. Total control over the ocean certainly sounds like it would appeal to the aquatic Naga and reminds us (on a much larger scale) of the actions of Lady Vashj in Serpentshrine Caverns.
The return of the Legion
There could be even more to it, however.
During his abortive attempt to usurp control of Undercity, Varimathras went through an elaborate ritual to allow something to enter Azeroth. Whatever this being was, it was so fearsome that Varimathras called it "master." Despite opening multiple portals throughout the throne room of the Undercity, Varimathras was unable to summon his master through before Thrall and Sylvanas attacked him.
Now, Azshara was responsible for the ancient Quel'dorei's attempt to summon Sargeras through the original Well of Eternity, an effort that nearly succeeded. Now, without access to the new Well atop Mount Hyjal, how could she possibly summon Sargeras through? Well, for starters, she's more skilled with magic than just about any living Azerothian, having had thousands upon thousands of years to perfect her craft (and she was considered one of the best spellcasters her people had when she was Queen of the Kaldorei).
But now consider that Neptulon seems capable of bypassing the Titan's wards that hold the elemental lords in their prisons. Where Ragnaros had to be summoned and was considerably weakened in that summoning, Neptulon simply seems able to manifest at will. Even before the Abyssal Rift was opened, Neptulon appeared to destroy the Gurubashi.
What is it that capacity that Azshara intends to make use of? She'd think nothing of making any promise the Old Gods asked and then subtly do her best to ensure their plans failed (while possibly steering the effects of the Cataclysm to maximize her own benefit) and once the Old Gods and their chief minion Deathwing were occupied with their destructive nonsense and abject chaos, she would be free to act and use Neptulon's special ability to travel between worlds to bring Sargeras back through to Azeroth again. It could take months, years, decades... but what is time to her, after all? And since no one but the servants of the Old Gods (who she expects to be abjectly defeated) knows she has the Tidehunter, there's no one to even come looking to stop her until it's too late.
Neptulon would not willingly submit control of any portion of his poiwer, of course. But if Azshara could twist Ozumat to her will (by the Old God's command, or her own power), then it may only be a matter of that same time she has so much of until he submits, willingly or no. And if he should, at the least the Queen of the Naga will rule the entirety of Azeroth's oceans.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.