I do not feel that the world stone was destroyed. If it were I do not believe that the evils would still desire the soulstones. I feel that Tyrael removed the corruption of the stone while at the same time altering it so that its’ diminishing effects on the Nephalem would be lessened. This enables the Nephalem (us) to manifest more and more power to aid in the destruction of the Burning Hells.
Soulstones- The lore is sketchy on a lot of issues but I believe that the creation of the soulstones by Tyrael and the Horadrim was well intentioned. They meant them to imprison a portion of the evil’s essence and in effect chain their power. This worked for a time while all three evils were imprisoned on Sanctuary. The soul stones were corrupted however, and because of this their purpose changed.
I feel that now the soulstones are anchors for the evils that when united with them, the evils are able to manifest their full power in circumvention of the world stone. Each of the prime evils got one. Each was destroyed in Diablo II when their associated prime was defeated and cast back into the abyss.
Tyrael- Everything we have seen of him suggests that he is trustworthy and has fought on the side of humanity, at times against the orders of the High Heavens. After his (alteration) of the world stone there was no further need of him for twenty years so he left Sanctuary. With the return of Azmodan and the Black Soul stone he returned to once again aid humanity against the Burning Hells. I feel that he was in the comet that crashed down. He was either returning willingly or being cast out forcefully from Heaven. He could either be restored to his full power, or even made mortal as a result of his possible expulsion. Either way I hope he plays a large part, I love this character.
The black soul stone- I feel that the primes were each defeated and their soulstones destroyed. I also think that another stone was created that has not been talked about as of yet. The black soul stone contains the essence of THE prime evil, a character that has not been mentioned. It either is a compilation of all three primes or a brand new overlord that is just now being put forth for us. This is the ultimate power, enabling whoever wields it prime status as well as power over the world stone and Sanctuary. This is why Azomdan wants it so bad.
Cain- I believe that Cain has been Belial from the beginning. It makes perfect sense to me. From Diablo 1 Cain has been instrumental in helping the Nephalem defeat the prime evils, his rivals. What would be more appropriate for the Lord of Lies and Deception to use the unwitting humans as his pawns to destroy his adversaries? This makes even more sense when you take the most recent cinematic into account.
Black Soulstone cinematic- This cinematic was a vision that Leah had which allowed her to see what Cain (Belial) had already experienced. Belial and Azmodan are the only evils left of the original 7. Belial had helped the humans kill each of the other evils. This is what Azmodan was referring to when he said that Belial thought he was so clever, and that each of the other evils had fallen into his (Belial's) trap. They are the last two rivals.
The last two evils are now aware of the existence of the black soul stone, their ultimate source of power and way around the deleterious effects of the world stone. So they will both seek out the stone for their own purposes.
Leah/Diablo-I do not believe that simply because Aidan was possessed by Diablo when Leah was conceived that this somehow means that Leah is part demon. A spiritual possession would not alter DNA and she has not to this point manifested any signs of demonic possession. I suppose only time will tell.
I do not believe that Diablo will come back through her though. To me it just doesn’t make sense. I think Diablo’s essence was destroyed with his soulstone.
(Diablo’s) Prime Evil’s return- Everyone is saying that the figure we saw was a female version of Diablo. I think that may be too simplistic. I think Blizzard may have referred to the ultimate boss as Diablo as a title perhaps. I believe that the black soul stone either holds the essence of all three prime evils, or it holds the essence of THE prime evil.
My further thought on this is why couldn’t the female demon be Lillith herself? It would make perfect sense to have to face her if human kind is finally to be free of the burning hells forever.