Diablo the Ally?

[Idomis]

From the Diablo 1 Manual:
"Since the Beginning, the forces of Light and Darkness have engaged in an eternal war: The Great Conflict, whose victor will rise from the apocalyptic ashes to hold sway over all creation... The Angels believe that only absolute discipline can properly restore order to the myriad realms, while the demonic denizens of the Burning Hells hold that absolute chaos is the true nature of all things.

...

Although the Great Conflict burned hotter and longer than any of the stars in the sky, neither side could gain dominion over the other for long. Both factions sought some way to turn the tides of the war to the favor. With the ascension of Man and his mortal realm, the Great Conflict ground to a mysterious halt. Both armies paused in a breathless stalemate, waiting to see to whose side Man would eventually turn."

Many of us have been speculating that the forces of Heaven will play an antagonistic role in D3. I agree with this. I think I speak for almost all of us, though, when I say that I don't believe the garbage that we don't know whether Diablo is going to return. I expect that he will return - I just expect us to do it for him.

With the three Prime Evils defeated (not utterly destroyed), Heaven is two Lesser Evils shy of being free of significant opposition. According to the D1 manual, their eventual goal is the control of all realms - ruling with the iron fist of order. This is highly undesirable for both Hell and the mortals of Sanctuary.

Speaking as a mortal, I personally desire a world with a balance between order and chaos. As a Sanctuarian(TM), I'd desire Heaven and Hell to be in balance. Now (crux:), I don't think it would be too far fetched to suggest that the Hero(es) of Diablo 3 want Diablo to return!

Problem: Heaven is poised and eager to invade.
Proof: D1 Manual (http://ftp.blizzard.com/pub/misc/Diablo.PDF) (Blizzard FTP), the events of Diablo 2.

Lore: Ureh is a gateway between Heaven and Sanctuary.
Proof: This (http://forums.diii.net/showthread.php?t=673352) link to Risingred's excellent research.

Perhaps only through some grand ritual can the Hero(es) of Diablo 3 restore order to Sanctuary. I fully expect (thank you, Risingred) that we will be traveling to Heaven in D3. It is perfectly set up. Perhaps there is some object or magic or location in Heaven that will be key to Diablo's resurrection.

A little extra speculation:
Cain can be seen here (http://www.diii.net/gallery/showphoto.php?photo=717&cat=563) with what appears to be a shard of the Worldstone. Perhaps we must reforge Big D's Soulstone with the Worldstone or maybe it's a weapon, defensive item, or key of some sort.

[Gamekk]

I always thought that it would end so we would need to fight both Burning Hell and High Heavens.

I can understand that you think we might turn into the side of Hell and bring down some angels, but rezzing Big D??? Sounds like what you would like too much to see in DIII.

Ureh theory is pretty interesting as well. Heaven already had their path to sanctuary, or a limited one (presence of Tyrael), so now we will also gain a path to High Heavens. It's most likely that we will step through that door so that some angels chills down and sit back. My guess is that it will be on the later acts, as the plot is revealing to us, we will understand that with Burning Hells being too weak, the High Heavens will try to take over the Sanctuary. I just wish it won't be DIII's expansion and that the plot will still be unrevealed.

[Idomis]

I can understand that you think we might turn into the side of Hell and bring down some angels, but rezzing Big D??? Sounds like what you would like too much to see in DIII.
We won't be allied with Hell necessarily. But if you have to amputate the arm to save the man, you do so. Likewise, if I had to resurrect/summon Diablo to defend myself from Heaven and re-establish a balance in Sanctuary, you can bet that I would.

If the only way to defend Sanctuary was to provide Heaven with a significantly powerful distraction, I'd do it.

[Ishmayl]

I don't see Diablo being any sort of ally - I think though that the heroes responsibilities in this game will be not to rid the world of evil, but to rid the world of absolutes, leaving Sanctuary in balance, which would definitely involve (possibly) fighting heaven AND hell.

[Apocalypse]

i think if mortals wanted to bring balance back between heaven and hell they would not do it by bringing back diablo but by taking down the powers of heaven. why help 1 faction gain more power when you can just weaken the second and push your own faction to the top of the food chain?

[mince pies]

Technically we (Sanctuary) are the top of the food chain anyway because there obviously isn't anything from the High Heavens that can defeat Big D otherwise they would've and mortals from Sanctuary have already defeated Diablo, Baal and Meph (Diablo twice over) so that means that we're pretty much unchallanged :) Would also be an interesting twist if at the end Sanctuary just goes "Ah, **** it" and obliterates both Heaven and Hell ;)

[Apocalypse]

now thats my kinda game. kill diablo, then when tyreal comes to say thanks, stab him in the eye(if you can find it)

[axxxel]

I like the idea of hell eventually becoming other than the enemy of the hero.

I think the two possible plots are based either on heaven turning antagonist or just tyrael turning (out to be) evil.

Or hey, here's another idea for the lamest sequel-save ever: Diablos soulstone was never destroyed and it corrupted the hero of dII and manifested himself in his body.