Pretty sure everyone that reads the story forums have heard the possibility of Leah being Diablo.
We know Azmodan and Belial should be some of the bosses and the Angaris (spelling?) Council was covered quite a bit so it's possible some of them could be final bosses too. Think it would be pretty cool if we get to fight them.
What if Leah is Diablo but isn't an enemy. In Diablo 2 it kinda seemed like the Lesser Evils helped the Prime Evils get exiled from hell. In the lore panel it seemed like the Lesser Evils were always against the Prime Evils. I could kinda see Diablo joining the humans to get rid of a more immediate threat. After taking out the last two Lesser Evils and some of the Angaris Council would make a great expansion for Diablo to betray the hero.
The lesser evils in Diablo 2 helping the prime evils could be explained by Azmodan betraying them after the Prime Evils getting kicked out to rule hell by himself but that doesn't really fit the video since he still called them brethren.
Diablo doesn't really have to be Leah for this to work which I hope she isn't.
So, what would everyone think if Diablo was an ally?