343 already said that they are not announcing Halo at this year's E3- unless I'm mistaken and am somehow confusing this year with last years.
Last years for sure...besides I'm thinking a 2018 release date because every halo has been released 3 years of each other with the exception of halo reach to halo 4 which was 2 years apart. This fall will be halo 5's 3 year mark so if they plan on continuing the trend it will be released this fall. I think they've been working on Halo 6 longer than most people think considering how much halo 5 has been getting neglected over these past 2 years while still holding most of the same problems since launch. We still don't even have a dlc map that can be used in HCS playlist yet. So yea I'm thinking since the foundation was built from Halo 5 most of it can just be ported over to Halo 6 with new campaign, new maps, new guns, vehicles, etc. making for an even faster release.