Straight from Hall H, Marvel Studios revealed during their panel that Evangeline Lilly will play Hope Pym the daughter of Hank Pym, and that Corey Stoll will play Darren Cross the antagonist of the film who is also taking up the YellowJacket mantle.
From the panel it seems that Hank Pym enlists the help of Scott Lang in taking down Cross who has was a mentee of Pym's and is now taking his science in a wrong direction. I'm also hoping that Evangeline Lilly gets more than just being Paul Rudd's love interest.
Also they test screened some test footage because they don't start filming till August 18th. Everyone has been worried for months on whether GoTG would be successful, I think we should have been worrying about Ant-Man. http://marvel.com/news/movies/2014/7/26/22968/sdcc_2014_official_evangeline_lilly_corey_stoll_join_marvels_ant-man