Threepio and the others will almost certainly be in the movie. As for the well-known characters outside the Troika, Chewie and the droids, who knows. I'd love to see Billy Dee return as an aging Lando and Denis Lawson reprise Wedge one more time, but I guess we won't know anything for certain for a little while longer.
There were the rumors that Judi Dench would appear as an elderly Mon Mothma, but who knows if that had any validity or not. I hadn't heard anything else about Dench being involved in discussions with the producers. That would be a nice piece of casting, though, if they were able to secure her for the role of the Rebellion's matriarch.