343 Industries Community Manager Brian Jarrard confirmed on Reddit that while there will be something Halo related at E3 2017, it will not be related to Halo 6. Stating..
“I’ll slip into my Dreamcrusher persona for a minute in the name of realistic expectations: We’ve said this already but we’ll have a little something at E3 but it’s not related to the next major entry in the franchise.”
Jarrard was also asked if the company was waiting until Gamescom which takes place a couple months after E3. Jarrard replied “definitely not.”
It is rumored that this announcement related to Halo will be an expansion for Halo Wars 2.