343 Industries has already said that Halo 6 won't be making an appearance at E3 2017, instead, the Halo developer will be showing off "a little something" else during the Microsoft briefing (which will be a bit longer than usual).
It looks like a YouTuber (Colteastwood) has shown off something that we might be seeing at E3. Below, you'll see what appears to be a GAME (UK based retailer) advertisement for something called Halo: Gravemind.
Gravemind is the second to last stage in the life cycle of the Flood and is essentially the controller of the Flood – it's the 'ultimate intelligence.' We see a particular Gravemind in Halo 2 as a side villain and the main antagonist in Halo 3.
In the bottom left corner of the poster, you will see 'Halo 6' which suggests that whatever Halo Gravemind is – it could give us beta access to the upcoming Halo game. The poster also details that Creative Assembly helped work on the game, the studio behind Alien: Isolation – which is fitting for the game's horror-like tagline of "This place will become your Tomb."
The director of the Halo series at 343 Industries, Frank O'Connor, has called the image fake, however, he also called Halo 2 Anniversary fake.
According to the poster, the game will release on October 10th, 2017.
**Take all rumors with a grain of salt.**