It's Starfield I'm like 99% confident.
They've got these stars in all of their E3 promotional material, Bethesda Softworks has grown a lot larger and I don't see why they would reveal both RAGE 2 and Fallout 76 prior to the show if they didn't have something huge left to reveal.
Between this and the stars behind the vault door on the Fallout painting for seemingly no reason it's got to be Starfield.
Don't forget guys that Hines very likely means other games published by Bethesda as well. Like a Doom 2,which would be fantastic of course. He doesn't mean BGS perse.
There IS a real good chance BGS will present more than just Fallout 76 though. Whether that is the long rumored mobile game, Fallout 3 remastered or a new IP remains to be seen. Daniel from Noclip clearly spoke about new games from BGS and it would surprise me if they aren't shown or talked about during BE3 and only covered in the documentary come June 12.