Empire Online
http://www.empireonline.com/news/story.asp?NID=38546
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Carter revealed that a third film would return to the franchise mythology and have a role available for Simon Pegg, if he were to choose to accept it.
“It’s really up to Twentieth Century Fox, whether they have the will to do it,” he tells Empire. “I think all of us are interested in putting the band back together. I have an idea for a third movie in my head. The colonization date has passed (according to the series the alien invasion was scheduled to happen on December 22, 2012), and that is something we wouldn’t ignore. For the second movie, we only had the budget for a standalone story, but we want to go back to the mythology.”