I think that "fate lies shattered forever" refers to the black soul stone being shattered and thus putting an end to the possibility of trapping the demons soul anymore.
Every other part of the prophecy refers to an angel, and there is a known angel of fate. We even met him. He (or something he is closely tied to, like the book of fate) is what will be "shattered" (possibly figurative.)
We already saw hope "swallowed" by despair. Everything else that we know in that prophecy appears chronologically in the exact order: Maltheal was lost long ago, then Tyreal falls, sometime after Rakanoth captures Auriel. Maltheal's reappearance as Death is the next step. There's one more step after for Fate, which coincidentally lines up with the 2nd planned expansion. So this means Valor --> Wrath already had to happen. How you want to interpret that happening is up to you; but there's no indication that pride for accepting help is what motivated Imperius, and mere hatred for half-demons trapsing around the heavens (Wrath) could well have motivated his actions. Humans aren't merely a "lesser" race from many angels perspective, they are a tainted one.
Fate is unable to read the actions of the Nephalim, not the *fate* of the Nephalim in particular. According to the book of Fate, heaven was already doomed once Hope (Auriel) was lost. But the unpredictable actions of the Nephalim could change that fate according to Ithereal, and they seemed to with Diablo being cast out. Thus the Nephalim's role in the future of Heaven cannot be known by the book of Fate; and with the destruction of the worldstone, more powerful Nephalim are running around now, some which may be antagonists. Adria is still unaccounted for, while Zoltun Kulle being "unkillable" is possibly still alive as well. And after main game -> Demons and first expansion -> Angel antagonists, some Nephalim harvesting their powers could make a good capstone and contrast for the 2nd (likely final) expansion.
Not that Ithereal necessarily *revealed* everything he knew from the book anyway, he may well have some foresight about the coming calamity that he just hasn't shown off yet.