So just what was Belial's plan there?

[Lissa]

Anyone else think it was a colossal misstep to morph Leah into Sheablo rather than Adria? As a plot twist, its shock value was limited to that first game you played through in Normal difficulty. But its spoiler effect completely sabotages any dramatic value of replaying the extended quest sequences in subsequent games. Now when I have to click through Leah's dialog to advance the plot, I just ignore her since everything she's admonishing me about is futile and her rudimentary character arc is meaningless. If Diablo had turned out to be a shady secondary character like Adria, it would have made just as much dramatic sense and been far more watchable in replay.

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.