I agree that Whedon would be a great fit for one of the new movies, he's signed to do The Avengers sequel which is supposed to be released in 2015 as well so I think he's tied up. However, one name that hasn't gotten a lot of mention that I think would do awesome is Joe Johnston. I thought he did a good job with the Captain America movie and he was one of the Art Directors on Empire Strikes Back (He came up with the Boba Fett design)
Joe Johnston might be a good choice, too. He also directed the underrated Rocketeer for Disney, as well.