Diablo III/WMG

The enemies of Diablo 4 will be Angels, not Demons
Heaven will declare war on Sanctuary, as humans are growing strong enough to pose a threat. The demons may attack as well, but they will not be the main focus of the game.
 * What makes you think you won't be fighting Angels in Diablo3?
 * Apparently this was originally going to be the plot of Diablo III but was scrapped early on, as a game set in heaven seemed "too different" from it's predecessors.
 * However, the  And there's the question of what would the expansions bring.

Dark Wanderer is Leah's father
We know Adria was her mother and a great warrior who was lost when Tristram fell to the demons was her father. This warrior was going to become or was already the Dark Wanderer.
 * An entry from Adria's journal as seen in the beta seems to confirm this
 * Makes sense when you consider that it's heavily hinted that Diablo will possess a female host this time, most likely Leah and that previously, everyone that Diablo has possessed is descended from King Leoric - the Dark Wanderer who was the warrior from the first game has been confirmed by Blizzard to be Aidan, the eldest song of King Leoric and Albrecht's older brother.
 * Confirmed.

Tyrael has gone rogue in D3 (added before any news released)
Either by taint of the World Stone, always rogue or having to flee heaven, Tyrael is now a rogue Angel. The first is obvious, and the second and third have evidence. Tyrael was the deciding vote on leaving Sanctuary in existence. He created the idea of using the soul stones on the Three Prime Evils. Then he destroyed the world stone after his soul stone idea fell apart. Either it was all part of an "evil" master plan or heaven will thing it was.

Adria will kill Leah and become Diablo's new host
It would be the perfect Player Punch.

Leah is currently possessed by or is Belial
Got this from a mix of the Black Soulstone trailer and the Book of Cain. In the Black Soulstone trailer it doesn't really make sense if Azmodan is talking to Leah. Either Leah's dreaming herself in the place of someone else, or Azmodan's talking to someone inside Leah. In it Azmodan talks about traps and deception, which is the providence of Belial.

Additionally, in the Book of Cain it says, "[Belial] is often trapped within the intricate webs of his own machinations." This could be forshadowing to Belial having taken human form to further his plans and having totally fallen for it himself, Becoming the Mask. He believes his own lie that he is a human woman named Leah.

Most of all, everything is screaming that this game's plot is going to focus on Azmodan and Belial. Leah being Belial or being possessed by Belial would keep things focused. And, well, continuing the family tradition of being possessed by Diablo is just too easy.

Jossed with the revelation that.

will be the expansion's Big Bad
The main page likened  to. So, following that plot, it may not be too much to guess that,. You thought the Angels were scared of the reunified Tathamet because ? Well, if. Of course, there's the problem of where to get the requisite power before Heaven catches on.

Adria may not be human.
It was noted how little she aged. She produced a powerful daughter. There is one lingering problem with Leah's parentage, though. Who is Leah's genetic father? It is implied Aidan earlier on, but later Diablo is the "real" father. Was this Albecht completely converted into Diablo or Diablo possessed Aidan who was still very human when the conception would have occured? If the former, this takes a hit. If the later, then Leah's powers (while guided to be hellish) would be from her mother potentially. Two possibilities, rogue angel or angel possessed human trying to recreate a Nepheleam as the potential ultimate host for Diablo if the plan worked. They weren't expecting, or this was Plan B if it didn't happen, the world stone's destruction unleashing the nepheleam's potential "naturally". If not an angel, then there is one demon who kind of just vanishes from lore, Lilith, who showed up as a bonus boss in D2. Maybe Adria was/is Lilith?
 * It's not certain what the rate of transfiguration of Aidan's body->Diablo's body was, but I wouldn't be surprised if, anticipating/guiding Aidan mating with Adria, Diablo made priority one the sculpting of Aidan's gametes, so they'd have at least some of Diablo's own aspects in there.

Malthael will prove to have become evil.
Although not necessarily by infernal taint. In any case, not only is Malthael missing,. And it's rather worrying, given that Blizzard is happy to use precepts from medieval Christian lore, how similar Malthael's name is to that of the Goetic demon Malthas (depending on the grimoire you use, you may better know him as Halphas). If Malthael is associated in any way with storks, the favored way for Malthas to manifest, we're going to have problems.

While the demons seen to date are explicitly not fallen angels, Malthael may start an all new sort of demon that is just such a form of malevolent Order. And he did say that he would prevail in the end, no matter what, when Imperius was calling for Sanctuary's destruction. Did the Worldstone's destruction force him to begin an emergency plan to guarantee Heaven's eternal primacy? Worse...is he a reincarnation of the original Anu, who purged Tathamet from himself and sought to obliterate him, and now is trying to finish the deed outright? Even at the cost of turning Sanctuary into an Eldritch Location by purging all of its demonic-source elements away?

Diablo III will have more than one expansion
At least two in fact, one dealing with  as the endboss and the other one dealing with Malthael, who may have turned evil. Both will probably use the  for their aims, prehaps resulting in a new Tathamet being reborn as. The Adria expansion may come first, given most player who actually pay attention to the lore's urge to kick her ass because of. The Malthael expansion may involve Imperius pulling a Face Heel Turn in order to destroy the Nephalem and becoming a major antagonist in the expansion.
 * But it's also possible that his battle will end not with his death, but with a Heel Realization that the Nephalem aren't hopelessly corrupt after all, indeed are closer to Anu's true original nature than the angels ever were.  As for Malthael's schemes if he proves evil...as I spoke of above, maybe him trying to restore and/or become a reborn Anu, even if nothing recognizable remains of the universe in the process?
 * There's . He survived the cutscene just fine, shows up at least twice in the game afterwards and even after, he and his friends are very clearly alive and moving.
 * For Imperius, it could be possible, assuming that he was killed, for his remains to be salvaged and imbued with the Black Soulstone. Think about it the most Badass angel merged with the Ultimate Evil! It would be a awesome battle!

The Monk's resource: Spirit is actually Spirit Energy aka Reiki

 * This would at least explain what type of Ki he/she is using.

The Real Money Auction House will fail

 * The RMH will not bring in the revenue blizzard expects, people will find ways of duping items, because really you can't stop the playerbase from figure out ways to do it, the economy of the RMH will be totally destroyed by it. Blizzard will flounder and bluster about the online single player being nessecary for several months afterwards until they realize people already have offline server emulators and THEN they will finally give in and stop screwing the player base.
 * Given that the items, like the characters, are actually kept on the servers, duplicating items is going to require server hacking this time. Even if Blizzard's intrusion countermeasures are imperfect, just how many of the earlier duplicators are going to be willing to take the time to hack the servers? It was planned as online-only from the first moment of inception, anyway?
 * Also, considering D3 has become one of the fastest selling games of all time, the idea that Blizzard will need to make money off the RMAH to pay off development is flawed. At worse, you could say that any money Blizzard makes off the RMAH goes towards server upkeep and additional budget for expansions. Mostly importantly, every game fanbase has this sort of reaction to this sort of thing and if it does happen, it's not because the auction house (or whatever) failed but because the game does.
 * Its unlikely that there's going to be an effective server emulation for Diablo III, given that the game client is incapable of running the game on its own. Any work done on creating an unofficial, hacked server is going to require a lot of reverse engineering and guesswork. Still, its entirely possible for a dupe bug to exist in Diablo III, just like there was in Diablo II on Battle.net.

Azmodan is in fact the greatest tactician among the lords of hell, because the standards in heaven and hell are very low

 * Being a skilled commander needs a number of qualities (ability to motivate your own forces, quickly see and respond to opportunities, have a good understanding of everything involved in the battle, etc.). Looking at the different qualities embodied in the archangels and major evils, the personalities and skills that would lead to these qualities are split between the angels and demons, thus, none of them would make a very good commander by human standards. Humans, who combine qualities from both Angels and Demons, have the potential to be much more tactically skilled, but against swarms of much more powerful demons or angels, this wouldn't matter much.

A future playable class will be bard
The terrible, Canon Dis Continuity Hellfire expansion for Diablo had three new classes: monk, barbarian, and bard. As of Diablo III, two of those are playable. In Act 1, at the inn, there is an NPC fixated on having a bard around to lighten the mood for everyone. He then decides that he will write the song about the (latest) downfall of Leoric himself, plus or minus trivial matters like truth getting in his way. The fact that the player character  lends credibility to the awesome power of BARDS! vanquishing evil with music.