The first Marvel movie of Phase 3 will be Ant-Man. We know that it will be directed by Edgar Wright with a 2015 release date. It will apparently be a standalone movie with a heist plot line.
Theses are the actors I think should be cast in the movie
Paul Rudd as Eric O'Grady/Ant-Man
Eric O'Grady is a low level SHIELD Agent who finds Technology created by SHIELD scientist Dr Henry Pym. Eric O'Grady is an almost irredeemable character due to his immoral actions. Over the course of the movie O'Grady begins to fix his behavior and become more off a hero. The recent rumor that Paul Rudd was going to play Ant-Man seems perfect. Rudd would make a better O'Grady than he would Pym or Lang. Rudd's Comedic talent and Wright's talent would make the movie very good.
Rashida Jones as Janet Van Dyne/Wasp
Rashida Jones looks the part and has very good on-screen chemistry with Rudd. You can't ask for much more really.
David Wenham as Dr Henry Pym
In this movie Pym would be a SHIELD scientist who invents the Ant-Man armor he doesn't actually use it. Wenham is a talented and underrated actor.
Not sure who would be the villain but since AIM had a small role in Iron Man 3 they could be the main threat in Ant-Man with Simon Pegg as the new leader of AIM.
Next I will post my ideas for what should (and probably will) come next in the MCU Phase 3: "Dr Stange: The Sorcerer Supreme"