Gears of War developer Cliff Bleszinski confirmed that the series was never intended to be a trilogy, going against what a lot of people at Microsoft have told people.
Gears of War "never was planned as a trilogy. Somebody at Microsoft said that and that got attributed to me. I think if you're the kind of person who says 'here's this game and it's going to be the start of an epic trilogy', gamers are like f*** you, let's see how good your first game is. Bring it. You put them on the defensive.