We killed Blood Raven (lots of times, too) I think she's going to stay dead.

Same for the summoner, and I think Diablo is going to be an enemy... although if D3 wil have humans fighting against the Heavens at some point, we might just have a few quests with the theme of "The enemy of my enemy is my friend" in which we work with one side against the other. Then, of course, they turn on us and we have to kill them as well.
As for NPCs/enemies that will be corrupted, I'm fully expecting to see some of the old Tristram D1 people to show up. Adria, perhaps in an evil role. Perhaps Natalya from D2 will make another appearance?
As for a corrupted Paladin... what skills would they use? I mean, the Paladin is a warrior that is empowered by whichever divine forces of good he serves. Some sort of Demonic Paladin should exists, yes, but how would he work?