No need for corruption, but they could divide our allies into Light-aligned (Velen, Anduin, Turalyon) and Void-aligned (Alleria, Wrathion, others).
Since Horde and Alliance aren't that relevant anymore, having these two opposite views on how to defeat threats could be an interesting replacement.
We already have warlocks and demon hunters using fel magic, we've got DKs, Sylvanas and the Lich King using the plague, I think these "fight fire with fire" or "no matter the cost" ideals could be the source of many gray-moral situations and villains. And not because some of them suddenly go insane, but because our leaders choose to declare war on them after they cross some line they don't agree with.
I see your point, and I sort of agree. Like I said, there better be a damn good story to justify this misdirection. In movies, when someone dies off-camera, there's always the chance of them coming back in a satisfying way. It can be predictable, it can be stupid or it can be very well done. Blizzard's writing isn't that great, we all know that, but from time to time they manage to surprise us.
I definitely disagree about most people not being OK with it ("nearly nobody"), considering most people don't know or care about the general lore, let alone details such as these. As for the few of us that actually read Chronicles, Wowpedia and such... we're nerds, we devour every new piece of lore that is thrown our way. We may complain about it later, but we're always wanting more.