I don't know, where things ended in 'Halo 3', there's a chance where the series would end.
And there is going to be a game that bungie is releasing called 'Halo Wars' where I think you control the marines, that also might end the 'Halo' series.
Apparently there's like 7 or 11 halo rings? Halo wars is basically like command and conquer. And there is going to be Halo:recon I know nothing about the release dates though.
Bungie is going to release a game titled "Halo Wars" but it will be more like a prequel because it goes back towards the start of the war, and is more like a real time strategy game, not a first person shooter.
at the end of Halo 3 on legendary you see the Master Chief dropping on a unknown planet (just search at youtube). i think that there will be a Halo 4 but i wouldnt be named Halo 4, but more like *title* a sequel to the Halo trilogie.