At a Microsoft press event the other day, game designer Cliff Bleszinski said Gears was not announced as a trilogy. Apparently, they only decided to make a second game because the first one did so well. A decision will be made on a third one when the second one comes out.
We'll go out on a limb and predict Gears of War 3 right now. For that matter, Gears of War 4, 5, 6, probably 7 and a racing game at some point.
We're crazy psychics.