I look forward to seeing some prequel characters in the new movies, I mean like Jedi ghosts. I don't know if you can have the other characters cameo unless they are played by other actors as they will look too young. I haven't seen very convincing makeup to make someone look older so that's probably the only way.
I know it should at lest be possible to bring back anakin and yoda, I don't know about having Ewan McGregor reprise Obi Wan tho, it will look out of place after Luke has only seen him as an old man so I hope they don't do that. I wouldn't mind seeing Mace Windu but Luke never met him so I don't know how they would introduce them. I hope they don't bring back Liam Neeson tho, his character was boring.