I would have liked Angel. I understand why it didn't happen, but I was hoping for a bigger Angel arc in S7 (I loved the episodes he was in though), and having The First show up to Buffy as Angel would have been really cool. I liked the scene she has in the graveyard with the vampire-psychologist, I thought that was really funny, but Angel would have been something else entirely. I think every one of the Scoobies got sucked into believing what The First was selling in "Conversations with Dead People" (only Willow really figured it out), but Angel showing up, and Buffy realizing it wasn't him, would have been awesome.
Beyond Angel, Cordelia would have been neat (although I'm not sure if she was dead at that point on the Angel series. I think she was, but I could be wrong). She would have been perfect for Xander. Jesse, yeah, he makes sense, although I'm not sure if he would have had the same impact as someone like Cordy would have. And for sure, everyone forgot about him. Xander must have some sort of superhuman powers for dealing with grief, because in one episode he's totally over Jesse.