Sometimes, it may look like we've cleaned house on all possible threats. The Lich King is dead, leaving the Scourge in disarray, the Legion was driven off, Deathwing is dead and the Aspects are out of a job. With the fall of Garrosh Hellscream's 'True Horde' on the horizon, we'll have dealt with the Sha and put paid to the legacy of yet another Old God. We're good, right?
Of course not. It would be silly to think that. Some of those threats I just mentioned are still, in fact, lurking around the corner. And there's plenty of other squamous monstrosities and terrible dangers lurking out there, ready to step up and take center stage and force not so reluctant adventurers to get their swords, spells or other implements of destruction ready. Azeroth, as has been pointed out, is a tiny oasis surrounded by perils.
So let's take some time and look at just what remains out there to potentially kill us, or enslave us, or eat our souls, or some variation of same. Who's left? Not all of these contenders have been hostile to us so far, but past neutrality or even friendliness is no guarantee of the future. Also, these are in no particular order - it's not a top ten list, it's a however many occur to me list.
Wrathion's not the only dragon to watch, though. The Aspects being no longer empowered by the Titans means that for once, they're merely extremely large and powerful members of their respective dragonflights - they deserve and command respect, yes, but they're no longer so capable of absolutely controlling their flights. The Blue Flight has already all but disbanded, the various remaining blue dragons seeing to their own interests, and there's no reason to expect otherwise from the other dragonflights. Indeed, we're more and more likely to see individual dragons simply taking action on their own, for their own purposes - while none are likely to exceed Deathwing in terms of power or influence, they don't really have to in order to be threats. A single dragon, concealing itself, nearly toppled Stormwind and brought the Alliance to ruin. What could dozens of hidden dragons do? What havoc could they wreak now that they don't have the Aspects to ride herd on them anymore? And remember, there's one black dragon equal in power to Onyxia or Nefarian still alive - despite his claims, Wrathion hasn't removed his elder brother Sabellian from existence. If Sabellian ever returns from Outland, he could likely claim and enforce dominion over the minions of the Black Flight still in existence. And I haven't even mentioned the former Aspects themselves - at some point, it's potentially feasible that Nozdormu will become Murozond, for example.
As powerful and potentially dangerous as the dragons are, there are plenty of other threats to be worried about. Besides typical ones - even after Hellscream's defeat, both the Horde and the Alliance have proved they can be great threats to the world - there are threats that may seem defeated, but which in fact could rise again. The Scourge, for instance, still exists. While its presence in the Plaguelands is diminished, the Northrend campaign did not remove the untold numbers of animate dead from Icecrown Glacier and while Naxxramas fell and the Scourge presence in Azjol-Nerub was diminished, it can't be forgotten that Bolvar Fordragon still sits with the Helm of Domination atop his head seeking to hold the undead legions in check. Should he falter, or worse, go mad the Scourge could once again go on the march. Also, there are those within the Scourge who retain some element of free will, and in the absence of a clear command from the Lich King these undead are seeking to carve out kingdoms of their own.
Also, one cannot forget the threat Sylvanas Windrunner poses - in command of her own faction of free-willed undead, and aware of the current division of power amidst the Scourge, she is a wild card that could in time destabilize the Scourge's current paralysis, for good or ill. She possesses the means to create new undead, has the very plague originally used by Kel'Thuzad to spread undeath (in an entirely new formulation) and may or may not possess the shards of Frostmourne. She has proved willing and able to abduct a member of the Knights of the Ebon Blade in order to essentially torture him into compliance with her will.
Of course, the Scourge are hardly the only necromancers in the world. The rise of the Risen in the former Scarlet Crusade bastions of power proves that the Nathrezim have a mastery of undeath that rivals the Lich King's, which makes sense, as the original Lich King was the result of Kil'Jaeden's magic working on the remains of Ner'zhul, the orc shaman who he had once manipulated into warring on the draenei. With the dreadlords of the Nathrezim able to survive death (Mal'Ganis even survived being killed by Frostmourne) it's possible that all of them still exist, and are biding their time and plotting against the world that balked their plans and dared to kill their physical bodies. This of course means that it is potentially possible that none of the dreadlords are actually dead. They could all be alive on Azeroth right now in some disguise or another. The danger there is staggering - the Nathrezim can inhabit the bodies of dead humans and impersonate them so flawlessly that one of them managed to corrupt an organization of paladins into a hate mongering fear group, and eventually tired of the charade, killed them all and raised them as a third faction of undead servants. And worse, with the dreadlords still potentially alive, and with Kil'Jaeden himself still nursing his black eye from his own attempt to invade Azeroth, this means that the Burning Legion's return to Azeroth is practically guaranteed. It's a when not an if scenario, just as Wrathion intimated.
Worse than that, however, is the fact that there are plenty of hostile, inimical forces of chaos and destruction that aren't part of the Legion, that we've never met or heard of or even seen yet. Sargeras spent untold time patrolling the universe and doing battle with hosts of such entities, and not all of them ended up in the Legion. The Titans themselves used to use some of these entities - the affinity of some warlock minions for summoning magic was originally used by servants of the Titans to find such summoners and do away with them. There is no reason to assume that there are not, in fact, entire contingents of demonic entities in existence right now who have never met nor heard of the Legion, waiting to stumble upon Azeroth. Furthermore, not only have we not really done much of anything to stop the Old Gods on Azeroth (the existence of Y'Shaarj's heart proves that even an Old God slain by the Titans themselves can still unleash untold devastation, as do the Sha) but there are countless other Old Gods in the greater cosmos we have no information on. Harbinger Skyriss' announcement "We span the universe, as countless as the stars!" might be hyperbole, but it also might not be, as the arakkoa's summonings in Shadowmoon Valley indicate.
Both the Legion and the Old Gods, then, stand as constantly present dangers that transcend their current presence on Azeroth, which itself may be far more active than it initially appears. After all, if a Titan killing Y'Shaarj barely slowed the Old God down, what chance is there that C'Thun or Yogg-Saron are out of our hair? Much less the not yet seen N'Zoth. Similarly, there are still whole armies of naga, led by their millennia old queen Azshara, who may act on their own interests or in congress with the Old Gods, and while we've slain two of the elemental lords, one has been kidnapped and the last remaining one, Therazane, is not terribly fond of mortals due to their role in the death of her daughter Princess Theradras.
There are more potential threats out there - Magatha Grimtotem and Maiev Shadowsong come to mind - but the point is made. We're still in very real danger. And that means we have plenty of stuff left to do, see and kill.
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.