Halo 3 is the end of the story of the Arc.
More 'Halo' games are bound to come, however MC won't be the main character or maybe not even alive.
*gulp* master chief dead, i hope not
They probably will make more halo games, it's just like oblivion they made different games:- knights of the nine, morrowind, elder scrolls 5: oblivion and shivering isles(although an expansion pac) all the same game with different story line. That could be easily done with halo