Eventually we're going to have to declare the Chronicles stuff common knowledge, but certainly not yet. Given the fact that the upcoming expansion is called "Legion," it really starts to look like this is going to be the definitive Burning Legion expansion. Given how the Legion has been behind most of the plot of the series - between the War of the Ancients splitting the continents, the exile of the Draenei, the corruption of the Orcs, and the birth of the Scourge, the Legion has had a hand in all sorts of major Warcraft developments. So where, then, does the series go if and when they are defeated at the end of their eponymous expansion?
Obviously, using comic book logic, the Legion will probably always exist in one form or another - Warcraft is a setting that's supposed to be able to persist in perpetuity. If killing Arthas didn't lead to the destruction of the Scourge or even the removal of a figure called the Lich King from the board, I don't think that we're going to see all the demons of the twisting nether all of a sudden neutralized as a threat, even if we do strike back against the Legion in a more powerful way than we have before.
First, in non-spoileriffic terms, we can talk about demons.
The Burning Legion has invaded plenty of times in the past, and while we've beaten them back, they haven't really lost anything they can't replace with only a couple exceptions. Unless Blizzard pulls a Mal'ganis on us, we know that Tichondrius is extra-dead. Illidan essentially reached in through the Nether to strike at the true demon behind the avatar that had been projected into our world, and Tichondrius is super-dead.
But this is clearly not something that is easy to achieve. It's possible the Titans had simply not figured out how to strike into the Nether itself when they started using Mardum as their demonic prison planet, but given how immensely powerful Titans seem to be, that suggests that what Illidan achieved is really unusual.
The point being that every demon we've slain to save our world has simply found itself back in the Twisting Nether, down a physical body, but otherwise no worse for wear.
Even if we do manage to shut down the portal at the Tomb of Sargeras, that means that we're merely closing the door and locking it at best - which isn't really much more of a victory than we've had in the past - the only difference being the scale of the Legion's invasion this time.
But let's say we somehow cut the Legion off permanently from returning to Azeroth. Are we done? Is Azeroth safe?
Short answer: Absolutely not.
Long answer? Let's go beyond the cut. SPOILERS AHEAD
So the obvious threat that has nothing to do with the Legion is the danger of the Old Gods. That shouldn't come as a surprise, but we've learned a lot of new information that explains what the Old Gods are and how they kind of make Sargeras look justified - or at least give him an understandable motivation.
Azeroth is a Titan.
Well, it will be a Titan. It turns out that Titans arise from planets that have something called a "World Soul." After a long amount of time, they emerge from the planet as a true Titan.
First off: We don't know what happens to the planet afterward. Ideally, the Titan kind of leaves like some sort of energy being, leaving the planet intact. Less ideally, the whole thing might crack like an egg, which might not be great for the people living on it.
The other issue is that while Azeroth should be plenty powerful if it does emerge, Sargeras was able to kill the rest of the Pantheon rather easily using his Fel magic. It's not clear that Azeroth would have anything in particular to help him/her in that eventual match-up.
Then there's the biggest issue: Azeroth is in serious danger of being corrupted by the Void.
The entire reason why Sargeras started his Burning Crusade was because of the Void Corruption he discovered tainting several planets. He first found the Nathrezim using this power, but he eventually discovered that there were powerful beings within the Void called Void Lords who sought to corrupt a World-Soul to create a something like a Void Titan. Their method was to send giant fleshy parasites to infest the planets in which they found World Souls. The parasites would continually burrow deeper into the planet and infuse the World Soul with the Void to corrupt it. Those parasites are what are called Old Gods on Azeroth.
Now, Sargeras' plan to combat this was to deny them a host. He went from planet to planet, destroying even the ones without World Souls out of fear that one might develop there. Essentially, Sargeras felt that if he destroyed everything in the universe, the Void Lords would be unable to grab a foothold, and something better might arise in the universe's place.
Now, the Old Gods cannot be easily excised. After discovering them, the Titanic watcher Ra and his armies of Mogu destroyed the most powerful of them, Y'shaarj. But not only did he unleash the Sha upon the areas of the world where he had been, his destruction also tore a wound open on the nascent Titan, creating the Well of Eternity.
It was for this reason that the Titans decided to keep the Old Gods alive - sort of the same reason you don't want to pull a bullet out of gunshot wound until you have a skilled surgeon ready to fix the damage.
But given that the Titans are dead, there's not really much in the way of maintenance being done on the Old Gods' prisons. We've faced C'thun and Yogg-Saron, but it seems highly doubtful that our victories left them permanently inert. And N'zoth remains basically unchecked.
By defeating the Burning Legion, we're rejecting Sargeras' radical plan to "save" us from the Void, but that still leaves the matter of Azeroth's potential corruption unaddressed.
It seems inevitable that we'll have to face N'zoth sooner or later. And even worse, while it seems as if the four Old Gods we know - N'zoth, C'thun, Yogg-Saron, and Y'shaarj - were the only ones sent to Azeroth, there's nothing that says the Void Lords can't send another one.
And of course there are plenty of other "lesser threats" that have been perfectly good justifications for expansions in the past. Indeed, when it comes to the Scourge, I've speculated that they might actually represent the tip of the iceberg for a power that could rival that of the Legion and the Void, as representatives of Death.
It'll probably be another year or year and a half before we have anything concrete on what comes after Legion, if anything, but Blizzard has given itself plenty of space in which to expand.