For returning characters, a MUST is See-Threepio and Artoo-Detoo. Even back in the day, Lucas said the only characters that would be in all nine movies would be the droids.
And Mark Hamil should be back as Luke. Luke should be the leader of the new Jedi Council and serve as the "old Ben Kenobi" role in the new film. I think Han and Leia would be more cameos in the first film. Perhaps Leia is the Chancellor of the New Republic.
The main characters should be new however. Since now that Star Wars is no longer the "story of Anakin/Vader," the saga should be related to the Skywalker family. Three trilogies, three generations. The new main characters should be two males and one female (following the Obi-Wan/Padme/Anakin and Luke/Leia/Han traditions) and at least one be a Skywalker, either an offspring of Luke's and/or a child(ren) of Han & Leia.
I could also see Luke dieing in Episode VII (mirroring Qui-Gon & Obi-Wan, of course) and being a Force ghost in the other two.
And here's something I could see that would really piss off the older fans. Do you remember in the Heir to the Empire book by Zahn, Obi-Wan appears to Luke one last time before passing on? I could see in Episode VII, they mention that Obi-Wan and Yoda have already moved on. And now, in this film, it is time for Anakin to move on, as he is proud of the work Luke has done. And, yes, that would be Hayden Christensen. (But, don't worry, it is basically a cameo).
I agree with most of this but the makers of the movie need to address the problems with the prequels rather than continue to make the same mistakes.
#1: No hokey politics. I don't want to see any of the Senate or any of that crap. In Star Wars it was pretty clear Lucas didn't really think out how their political system worked. It's an Empire with an Emperor but theres an Imperial Senate, with Regional Governors. Now this doesn't make a whole lot of sense but we don't see any of it and it isn't explained so we can assume we just don't know how it works. In the prequels we see the Senate and the politics are extremely convoluted, the whole thing is a big room full of aliens that have no military and applaud at everything anyone says. In the sequels I don't mind seeing hearing about the Senate but I think seeing them or making Important characters like Leia in charge would just make the universe feel too scripted and small.
#2: I don't want to see a million Jedi, but if we have to, I don't want to see another Jedi Council. The big problem with the prequel Jedi is that they are some creepy cult. The kids are taken from birth raised in a sterol environment, hell in the EU it seems like once every character reaches puberty they have some sexual crisis, what happened with Anakin was only a matter of time. In the originals being a Jedi was so much more a choice, when Ben Kenobi tells Luke about how his father wanted to go with him on his adventures, then tells Luke "come with me", it seemed like more of a call to adventure than anyone could do. The biggest blow to me however was the Jedi Council, I love referencing them and they have become an iconic part of Star Wars, but it was a collection of the greatest Jedi in the Galaxy...sitting in a room debating. I mean is that really the Jedis greatest goals, not going on adventures, but to sit in a room and argue with other Jedi? The Jedi need to be cool again, more like Luke in Jedi, Ben in ANH, Qui-Gon, and less like Mace Windu, the boring no name characters that die in two seconds in Sith.
#3: I don't want to see Hayden Christensen as Anakins Ghost, for one he didn't know he was going to be put in Return of the Jedi. The footage was from a costume test for Revenge of the Sith and Hayden was getting his hair cut. You can't see because it's from the front but Hayden actually had long Qui-Gon esc ponytail in the source footage. Not to mention it makes no sense he look like that, because isn't that evil Anakin? Isn't that the Anakin that fell to the Darkside, that murdered children? Shouldn't his ghost be the good Anakin, a 40 year old male? Recast him with someone more age appropriate and someone that resembles Sebastian Shaw in that unmasking scene. I honestly have nothing against Christensen as an actor or that change, I just think it doesn't make sense and it was more of a promotion for Episode III than anything else.
#4: The new movies need to have more practical effects and sets and less clean green screen. Everything felt so fake in the prequels and the sets/props/costumes made it feel like a different universe at times. Like the lightsabers had big red buttons and looked more like toys than weapons, there was too much stuff going on in all the frames it looked too busy and cluttered compared to what we saw in the originals.
#5: We also need to have the cast return. Mark Hamil as an important mentor to the new hero. Some new non force related characters to balance out the magic and fantasy elements. Maybe Harrison Ford and Carrie Fischer in the later films to avoid making the universe seem small. But the big link should be the droids R2 and 3PO. Start it like A New Hope, that movie would be nearing it's 40th anniversary, to copy the plot to some degree would not seem as much of a rip off as paying homage at this point. Copy the small elements but make it something of it's own.
Anyway thats my two cents.