So just what was Belial's plan there?

[ViralSpiral]

I think Gillian would have been the best choice, especially after the two lore comments from her journal where Adria is to help her with her nightmares. If Adria had Gillian in tow and Gillian then became Diablo's vessel and Leah became the new head of the Horadrim and rebuilt the order, that would have lead to a much more interesting story and potential permutations down the road. Instead, we now know that Adria will somehow get the black soulstone and bring Diablo back into the world somehow.

Pffffft. "Hello, how may I burn you?"