Just my .02 from a fan that enjoys Diablo. I haven't read the Book of Cain or anything outside of the games(storyline/cinematics)
As far as "expansion content"
1) Immediately the disappearance of Malthael raised an eyebrow. Even in writing outside of video game story line, the sudden disappearance of a character(of such importance, even though we as the "player" haven't gotten to see it through d1-d3) usually means a significant change in them and how they impact the story line. So I think like everyone's been saying "good turned evil" will definitely be a possibility. The other possibility is that he took a leave of absence in search of knowledge(wisdom) of how to defeat whatever comes next. Why wouldn't he tell anyone especially the council of what he was doing? I don't know but to me that would be the more interesting of the two, that is instead of the age old "good turned bad" it's something else, especially when we're all anticipating he's gone evil.
2) I don't know about or care about Tathamet, I've killed enough internet dragons playing WoW, however I think a monstrous beast of absolute pure evil would be pretty intense(like bigger than Arreat when Azmodan shatters it). Also with this "being" coming back into existence, I think we'd see the return of Inarius and Illith thus allowing more "fresh" story. However, from what I saw in the end cinematic it was the black soulstone that fell back to Sanctuary upon defeating Diablo. If we assume that us as players/mortals defeated azmodan/belial(and all the rest, mephisto/baal/andarial/duriel etc) I'm also in agreement that on the fall Diablo's soul went into the soulstone. This is now a black soulstone with all seven of the evils(4 lesser 3 prime) in it. This is a catalyst for something which I think will come into play in the expansion. The only people (to my knowledge) that either know about it, or its power are you as the hero, leah, adria, and the angels. So I think assuming Adria's character doesn't die in d3 she will somehow get her hands on it, and use it for "something". I don't think or believe it will be releasing the evils from it, but rather using its power for something. They can only repeat the cycle of "soulstone this evil, it gets loose, defeat the evil, soulstone, repeat before it gets boring, and while that's the main theme throughout diablo, I feel like blizzard is trying(repeat: trying) to introduce depth(which is ironic because I think d3 franchise is one of the most in depth story lines of all of their franchises) to the story thus the introduction of Leah as a character that is supposed to represent hope.
3) I think Leah as a character will make a return somehow, whether through us redeeming/resurrecting her, or some other device. Now with Deckard Cain gone, the hero is in need of someone to show them the way and accompany them, I think Leah will fit that role quite well. However, after everything that happened I think the Leah we knew, won't be the one that comes back, but rather a darker twisted husk of a soul after having Diablo possess her. However, this will give her insight into the plan of the Prime Evils(and how best to aid the hero in stopping it).
4) Despite it looking quite dire, I don't believe Imperius dies from his wounds inflicted by Diablo. Even if he did, I find it odd in terms of story that you(mortals) can kill prime evils, but they just eventually come back. So if you reverse it and have a death of an angel of such importance, that they can't come back. Otherwise it would seem the odds are in favor of the forces of evil because once an angel is killed they're actually dead. Like in the Wrath animation clip, Imperius "kills" Diablo and he just comes back, so for it to be reversed and Imperius unable to come back would seem like a major **** up in the rules of the story. I do however believe that we'll see Imperius return much more for wrath than valor when he does come back. I don't believe the animation short was intended to fulfill the line of "Valor shall turn to Wrath", I think we have yet to experience the wrath of Imperius. I would argue that Imperius holds Tyrael responsible for Diablo reaching his "shining heaven" and defiling it. Also this ties into my Malthael one, if Malthael is going down the road of "good turned bad" or is even on the fence, I feel like Imperius would convince him to join his cause, implying that the Angiris Council is tainted by the actions of Tyrael(blasphemy when he ripped off his wings etc).
Idk, some stuff I was thinking about, when I wrote it I rambled on a bit. No TLR version sorry ^^