I like that last paragraph. Chewbacca, R2D2, and 3PO are absolutely NECESSARY. Chewy is just such a likeable character, all furry 7 feet of him. And R2 is the coolest droid to ever bless the big screen, his adventures within the star wars plot are key, and his friendship with other characters such as Anakin and then Luke make him another important member. C3PO can be a little whiny sometimes, but overall, I like him, and he is a good buddy for R2 and adds that touch of comic relief I suppose (he's not that funny but he does lighten the mood).
Also, regarding which character(s) I want to see return, it is hard to say. Most of the cool people are all dead, or already expected to return (Han, Luke, etc.). I can't even think of anyone else. Maybe Darth Maul, who is still alive by the way, according to the official "Star Wars: The Clone Wars" TV show. Or perhaps if we could see the return of the trandoshan bounty hunter known as Bossk. He is a character that many fans probably don't even remember, but I thought that he looked interesting in episode IV (I think it was that one, or episode VI), and he also was given more of a background in the Clone Wars TV show.
However, the timeline really takes a toll on the movie already. The fact that actors like Mark Hamil are return is exciting on one hand, yet a bit of a let down on the other. I mean, he has put on weight and years. Having a main cast of people on the older side who haven't preserved themselves seems like a more awkward Star Wars. On top of that, the actual universe will have to have aged in the same way as the actors. Meaning that a character you liked in the Clone Wars TV show should be about 100 years old by episode 7's time period.