I really hope there won't be any old characters coming back as force ghosts. That was one of the things I really liked about Zahn's books - he recognized the stupidity of the scene in ROTJ, where Obi-Wan, instead of being a disembodied voice or a flickering apparition, appeared as a ghost to rest his but on a stone and have a whole expository conversation with Luke and so Zahn had Obi-Wan appear to Luke one last time telling Luke that he's moving on and Luke will have to deal with what's coming on his own.

It really cheapens the death of a character, when you can just bring them back whenever it's convenient - it kinda worked in ESB, because it was handled pretty tastefully but I think that on the whole, it's a pretty cheap plot device and it would be even worse to use it just to give the dead characters some kind of a cameo. Plus, imagine the horror if they brought Hayden back as Anakin's ghost... Urgh!