Take a look at what a source known as "Jedi Master SQL" had to say.
While this insider wouldn't reveal any specifics regarding the plot, the source did hint that there will be a planet which has been previously mentioned but never seen on screen in Star Wars: Episode VII, which was added in the video game Star Wars The Old Republic.
When asked if "The Big 3" - Mark Hamill, Harrison Ford and Carrie Fisher - have signed on yet, SQL revealed that only one of them has signed on thus far.
All things said, all of them have worked with Lucasfilm in the last few years in one avenue or another - and all bring something interesting to the mix. But you need something of substance to carry off having them back; otherwise it becomes an awkward task of making a meaningful cameo for them.
It would not surprise me if they did not all come back for Episode VII but maybe came back at varying points in the new trilogy. The script will determine their status, but the script must be story driven, and if the natural story includes them all I will be ecstatic - but I don't want a script to be written to ACCOMMODATE them all. If they are back, it must be of substance.
I would suggest that the whole principal cast would be announced in a group. It creates a natural shield of scrutiny for any young or new talent (or at least delays or spreads it out) as everyone has a mass of stories to write at once. Judging by the history of PR releases to date, this may be a simple black and white press release on The Official Star Wars Site with a photo of the group."
In related news, a source close to the production at Pinewood Studios revealed that the crew has already constructed a full scale model of the Millennium Falcon, which is supposed to be even bigger than it was in the original trilogy.
Of course, nothing has been confirmed by Disney or LucasFilm yet, and it isn't known how this iconic ship will play into the story line.
Star Wars: Episode VII comes to theaters December 18th, 2015 and stars John Boyega, Daisy Ridley, Carrie Fisher, Harrison Ford, Mark Hamill, Adam Driver, Domhnall Gleeson, Lupita Nyong'o. The film is directed by J.J. Abrams.