I really, really can't see Spike fitting in at all.
Connor would be great, but only if he comes back as a completely different person, who doesn't remember any of season 3/4. He seemed cheerful in the last moments of season 4 and he would be much more fun like that. The show is dark enough already.
I already said this in a different thread, but if he was introduced properly, he could take it very well. I would be pretty impressed if someone came up to me and said:
"You're a superhuman, and we need your help to save the world" or something similar.
Also as *Lost Angel* says, the show needs a new approach, and this would be a better way to get one than just bringing over an old face from Buffy.