Adam And EVE IT: Sandbox FPS Project Legion Announced | Rock Paper Shotgun
CRYSIS 3 Set in New York City, Features Sandbox FPS Experience and Launches in Spring 2013
OpenSpades Sandbox Building & FPS Game FPS Games
FPS in The Sandbox - The mechanics are already there, and rumor from the livestreams is that they are getting updated. You can check out some examples of it already though! :