Marcus Fenix's story will conclude in Gears of War 3, but the game's universe will go on.
Since the very first trailers for Gears of War 3, Epic has been promoting the game as The End for the COGs and their support. This is it; this is where it all finishes. Speaking with CVG, Gears 3 executive producer Rod Fergusson confirmed that this was a game designed to give fans the grand finale they were hoping for: "[Gears 3 will] answer all of the questions, bring the story to a closure and go out with a huge bang."
However, that doesn't mean that the universe (and rather lucrative franchise) of Gears of War 3 is over and done with. "We're not blowing up the world so Sera and the Gears universe will live on," said Fergusson, though he stressed that Epic was aiming for an ending that fans would be satisfied with.
"[We] didn't want to betray our customers and community by having a cliff-hanger ending where you're like 'argh I just got duped, now I've gotta wait until the next game,'" he explained. "We're not going to reveal everything because we want some mystique left in our universe, but this is the Marcus Fenix story and people should feel satisfied after playing through that."
Epic could perhaps take a few pointers from fellow Xbox-360-exclusive series Halo. 2007's Halo 3 neatly concluded the saga of the Master Chief (albeit with a post-credits sequel hook), but Bungie went on to tell two more stories in the Halo universe with ODST and Reach.
Hell, I think a Gears prequel about Emergence Day could actually pretty cool, now that I'm giving it some thought.