So hear me out. I predict there will be two diablo 3 expansions (or possibly one obscenely long one). The way I see it, there are 4 loose ends to diablo 3 : Adria is missing, Black Soulstone was carelessly thrown off a cliff, Imperius hates humanity, Malthael is still missing. Here's how I think it goes.
In the aftermath of Diablo 3, your hero is looking for a way to revive Leah or restore her soul or something, so you set out for the only thing you know is linked to her: The Black Soulstone. After act 4, I don't know where it fell, but I assume it was Pandamoneum, the area between heaven and hell. So in act 5, you go through the world looking for a way to access pandamoneum, and when you finally find it, Adria pops out and is like "You led me to the last source of Evil in the universe HAHAHAHAHAHA so she runs in ahead of you after summoning demons and shiet. So after going in, you realize that the demons that are still in hell are trying to bring the Evils back alongside Adria, so you must fight your way to where the Soultone is sitting. So you eventually find it, but Adria is there opening it. She transforms into a demonic form she developed from being in contact with the stone and you fight and kill her, but she's like "It's too late! I've already opened it. BWAHAHAHAHA!" So now you do some crazy Uber Tristram style battle where you kill the evils in pandemoneum. It'll be Andariel and Duriel in the first room, then Azmodan and Belial in the second room, then finally you do a room where you must consecutively battle Mephisto and Baal (Diablo too, but he may be dead from the act 4, not sure how it works). You beat them and you've defeated all evil in the universe forever... Or so you think.
Act 6: Starts with the Angiris council discussing stuff about their treaty with the humans or something, and Imperius leaves cause he still hates humanity. He goes off somewhere and starts breaking stuff out of anger or something, when suddenly he hears a bone chilling "It's been a while, Imperius". Malthael reveals himself to Imperius and Imperius is like "WTF YOU'RE ALIVE" and he's like "yeah, I look more evil and badass too. I know why you are restless. Why you feel this wrath. The imbalance; The imbalance of the nephalem. They haven't entirely unlocked it again yet, but they are still too dangerous for our kind. They are the creations of heaven and hell, they may have good qualities but are inherintly greedy and will try to destroy our kind." Imperius is like "!@#$ dude, what can we do about it" Malthael gets a badass monologue and is like: "Since I have lost the name of wisdom, I would like to think I now stand as the Archangel of Balance. In other worlds in the universe, though, I've been called the Archangel of Death, because there can be no light without dark, no north without south, no life without death. We must eradicate humanity." And that's how act 6 starts.
Just when you think there's peace, Imperius and Malthael do something like unlock the dormant armies of hell and declare war upon humanity and heaven. This act works like act 3 maybe, and starts with hell invading Sanctuary, but as your hero defeats them, you push them back into hell, where is where you go to to finish them off. In a final epic battle, Malthael kills Ithereal (fate lies shattered forever) and Imperius is like "woah, we're going against the nephalem, not angels" and Malthael is like "they sided with the humans, they decided their fate". Imperius starts to have regrets and stuff. You are fighting Malthael, and as you're fighting him, part of his battle will be he stuns everyone and ais about to kill them all, when imperius turns on malthael. Malthael kills hims too, but not before he is weakened. Your hero then kills Malthael and is overcome with nephalem energy. Your hero becomes so powerful, you decide to sacrifice yourself like Uldyssian so that you may return the universe to balance and peace, and maybe you see Leah's spirit or something and she welcomes you to death. And they all lived happily ever after.
Again, tell me what you think and leave your won ideas below. Who knows, maybe Chris Metzen will read this (one can hope :D ).