According to some sources, one being Gamespot, Halo 6 won’t be shown at E3 in June. People can speculate as to why this could be happening, but rest assured that it has nothing to do with the development of the game, or any issues you may think it has. The only reason that truly matters is that, according to Brian Jarrard, the Halo Community Manager, there won’t be a new Halo game coming out in 2017. In other words, there’s nothing to showcase that has to do with Halo 6 at this point in time.
Instead, I believe Microsoft will use this opportunity to focus more on the release of Project Scorpio, set to hit stores by the end of the year. There’s a lot of speculation going around as to what exactly Project Scorpio will compare to, or if it will be in a league of its own. E3 will be a good time for Microsoft to show people what they’ve been working on and a little smidgen of what sort of games we can expect to see come time for its release. There most certainly won’t be a Halo 6 by the time Project Scorpio is released, but maybe that’s the point. It’s a possibility that Halo 6 will be released exclusively on Project Scorpio, much like Halo 5: Guardians was for
As of now, there isn’t anything set in stone as to what Microsoft has planned for E3.
Stay tuned for any more news regarding Microsoft, E3, and Halo 6. Considering E3 will begin on June 13, there’s bound to be more information on what Microsoft is planning for the event in the near future.