One of the theories I've seen online is the Carmine Theory about how many Gears of War games there will be before the story is completed.  In the first Gears of War we were all introduced to Anthony Carmine, the green as grass rookie in Delta squad.  Anthony gets taken out by a sniper while complaining about his gun jamming in the first act of the game.  A. Carmine was always yelling over the top stupid stuff and became a fan favorite in multiplayer.  In Gears of War 2 we were introduced to Benjamin Carmine, the green as grass rookie who is the brother of the first Carmine.  Ben seems a little bit more competent but still buys it in the second act when he is eaten alive.

From the game we know there are three other Carmines running around out there.  There are still two more brothers of Anthony and Ben with a third, Madison Carmine, who is noted as a war hero in the Jacinto Sentinel Times.  We know for sure one brother is named Clay from a note found on Ben's cog tags.  Notice a pattern?  Anthony, Ben and Clay?  I think it's safe to assume Clay will make an appearance early on in Gears of War three and die in the third act of the game.  I also think it is safe to assume we will be meeting a D. Carmine in the fourth Gears of War game.

Very early on Gears of War was pitched as a trilogy to Microsoft.  Everyone has been assuming that the franchise would have three games similar to Halo.  Still the lead designer behind Gears of War has spoken out against this idea.  If you have a hit game why limit yourself to just three games?  So far every Gears of War game has excelled so I don't see why they would stop at three if they can make four.  The Gears of War universe is large enough to have a couple spin offs as well.  I think you will not see the end of Gears of War on the Xbox 360.  I expect the game will make it to the next generation of Microsoft gaming systems as a possible launch title.

Source: Gears of War Wiki