I received my copy of the Book of Cain last week and while I have only thumbed through it and read the first 7-8 pages so far, I did come across something that was quite interesting to me. At the beginning of the book Cain has written a note to Leah and in this note he tells the end time Prophecy, the same one which is spoken of in the D3 beta campaign. Here is the prophecy as it's written in the book:

...And, at the End of Days, Wisdom shall be lost
as Justice falls upon the world of men.
Valor shall turn to Wrath--
and all Hope will be swallowed by Despair.
Death, at last, shall spread its wings over all--
as Fate lies shattered forever.

Now as I was thumbing through the Book of Cain I looked at the section concerning the Angels, including Tyrael, and noticed that each Angel is tied to a virtue or trait, the same ones that are listed in the prophecy. It would seem to point to a very grim fate involving the Angels of Heaven. I'll add the Angels names into the places where they are referenced in the prophecy:

...And, at the End of Days, Wisdom (Malthael, Archangel of Wisdom) shall be lost
as Justice (Tyrael, Archangel of Justice) falls upon the world of men.
Valor (Imperius, Archangel of Valor) shall turn to Wrath--
and all Hope (Auriel, Archangel of Hope) will be swallowed by Despair.
Death, at last, shall spread its wings over all--
as Fate (Itherael, Archangel of Fate) lies shattered forever.

This would seem to point out that Tyrael is the meteor that falls into the cathedral (as has been speculated) and the rest of the Angels either become broken or eventually fall in battle to the powers of Hell. Imperius it seems may actually become corrupted like Izual or betray the heroes and Heaven.