Looks like Microsoft has big, extensive plans for the Halo franchise. Earlier yesterday, on the 14th, Microsoft snagged the domain names for Halo 7, 8, and 9. Check it out:
http://www.complex.com/video-games/2012/09/microsoft-registers-halo-78-and-9-domain-names
I guess this means that Chief won't go out with a bang in Halo 6