Joss Whedon is out of the question at least for Episode 7. He's currently busy with Avengers 2, so there is no way he could direct a movie the size of Star Wars and have it ready by 2015.
The same goes for another rumored director, J.J. Abrams, who is tied up making Star Trek 2.
That doesn't rule out either director for Episodes 8 or 9, however.
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