I think it's pretty obvious that the one "ultimate evil" of the warcraft universe always has been, and always will be, Sargeras.
This guy has had Azeroth on his !@#$ list ever since he was metaphorically %^-*! slapped back into the Well of Eternity during the War of the Ancients. He's been pretty determined to tear us a new @#$%^-*! ever since.
Across the last couple expansions, we've seen an increasing amount of new lore about the Titans and their constructs. Almost as if, not surprisingly, they're building up a big reveal of some sort that has to do with the Titan Pantheon, which we've already known for a long time Sargeras is a former member.
I want to point out, just to support Sargeras as public enemy #1, that just about every major event was the result of Sargeras' grudge.
So Sargeras tells Kil'jaeden to come up with a way to weaken Azeroth so he can just kind of take over. Of course, Kil'jaeden's bright idea was to turn the orcs into bloodthirsty warmongers and let them have their way with us. Meanwhile, Sargeras has already tricked Aeqwynn into "killing" him so he can hide in her unborn child.
So there's the First War, went pretty well until the orcs started killing each other instead so that plan was a bust.
Kil'jaeden's next great idea was to take this guy Ner'zhul, stuff him in a block of ice and armor, and throw him at us. Pretty good plan. Can't stop a demonic invasion if everyone's a zombie, am I right?
Of course, Ner'zhul was pretty pissed at the whole frozen in ice forever thing (having his soul ripped to pieces probably didn't help) and that plan went south real fast seeing as he really didn't care about it in the first place.
I imagine at this point Sargeras is pretty pissed.
So here's a little something I noticed in game, and I'm not sure if anyone else has.
When you first meet Wrathion at the Tavern in the Mists, he gives a nice little speech about how some horrible darkness is coming and we're all doomed if the Alliance and Horde don't set aside their differences to stop it.
He also shows us a cute little hologram of Azeroth. With giant green asteroids hurtling towards it. Yeah, those are infernals about to rain down on us. Exactly like the opening cinematic for Warcraft 3 Reign of Chaos. Remember that one?
The second arrival of the Legion, and this time Sargeras himself, is clearly imminent. Really, I feel like it couldn't be more obvious unless the CDevs went ahead and spelled it out for us in a blue post. We could probably plug up the Maelstrom with all the npcs that like to rant about the upcoming darkness and impending doom.
With this last expansion we'll see, I hope, a couple things happen. Aegqynn will return and play a big role in Sargeras' downfall, mostly as repentance for the whole Medivh/Dark Portal/Orcish invasion thing. At least one of the Titans will return, most likely Aggramar seeing as he's taken up that vacant position of Titan champion.
Here's what will definitely NOT happen:
The Emerald Dream- Sorry, this little mini story already has an ending and there's nothing they want to do to continue it. The conclusion is in Stormrage, a novel by Richard A. Knaack, a really good read that I wholheartedly recommend.
Naga (maybe)- This one is a little bit of speculation on my part. Cataclysm attempted to end this storyline, but there must not have been enough time for a full conclusion. There's a raid portal right next to Throne of Tides that was never used and probably would have been to kill Ozumat and save Neptulon. I feel like the final expansion wouldn't be the right setting for these guys anyway. The Shattering was the perfect opportunity but now it's gone. The Sea Giants seem to have them under control anyway.
A hundred apologies for the wall of text no one will ever read, but the tl;dr is pretty much that there most likely will only be one more expansion, Sargeras is coming, and why there simply can't be anyone else more fitting for the end of the franchise.