Rumor: Harrison Ford, Mark Hamill and Carrie Fisher will return for Star Wars - Episode VII
Plausibility: High
Let's get real for minute here. Anyone looking to make a new Star Wars movie wanting Luke, Leia and Han to come back for guaranteed cheers in the trailers and big splashy press interviews about "coming home" and such is not news. Of course Disney wants that, and of course those were among the first three phone calls anyone working on this was instructed to make.
As for whether or not any of them would actually do it, with the exception of Ford (who only shows up for something if he actually wants to, at this point) I'd say chances are pretty damn high. Yes, Hamill and Fisher are both far from the poor house, but they (and their families) could certainly use the mountains of gold they can demand from The Mouse to walk across a green screen and give the Jedi Generation a final triumphant wink. Also, all involved seem to be much more at peace with the franchise than they were even a decade ago, and being feted like red carpet royalty (maybe taking a cast bow to a standing ovation at that year's Oscars, for example) would make for a pretty excellent coda to career stories that currently end on "eventually re-invented himself as a sought-after voice actor" and "ongoing struggles with drugs, alcohol and other issues," respectively. Remember, everyone from Star Trek was pretty much considered a has-been until they started making the movies.