I've always sort of wanted to run a campaign that begins with a bunch of evil PCs being killed in the first game. Their souls wind up as conscripts in the Blood War for a few decades or so, and then, due to some outside interference, they are given the opportunity to escape into the planes, and eventually (despite technically being dead) back to the Material Plane. There, they not only have to dodge the fiends who are searching for them, but they are forced into doing good--despite their initial evil alignments--because every evil act they commit draws fiendish attention to them, while good acts tend to deflect it. By the end of the campaign, those who have truly reprented, and now believe in doing good for the sake of doing good, might be allowed a second chance, while those who haven't would probably be recaptured and dragged back to Hell. (Or not. I don't predetermine the ends of my campaigns; I just like to have an idea of where it'll go if the PCs don't drastically change things.)
Never actually got it off the ground, and it's going to require a group of players really ready to get into it, but I think it could be fun.