This has already been answered in http://www.gamesas.com/index.php?/topic/1098305-fan-interview-pt-2/
Will we be able to keep playing once we beat the game with this one?
From Sean via Facebook
JS: This is something we really wanted to do, but ultimately we realized that supporting post-endgame content would jeopardize the quality of the ending, which we wanted to tell the definitive stories for all of our major factions, locations and characters. Instead, after the credits roll the game will prompt you to reload a save created just before the endgame sequence, allowing players to go back and complete any quests they may have missed. Additionally, we make it very clear when you're about to reach the end of the main plot, so it shouldn't come across as a surprise.