What's one thing you think will be in the game, despite having no reason to think so? It's all fine and dandy to see a detail in the E3 presentation and base speculation on that, but the previews Bethesda has released certainly don't even scratch the surface of what's actually in the game. Fallout 4 is, now, a big old radioactive iceberg. What do you think is below the surface?
I'd say Talon Company. Bethesda introduced them in Fallout 3, and they were a huge presence in DC. However they didn't seem native to the area; they only had small outposts here and there, aside from their base which wasn't all that impressive. They're also a random encounter in Shelter - what's more, they can be friendly! I think they're a prime target to explore a previously shallow idea in more depth.
Also: actual radioactive icebergs. What do you think?