Let me say that I love Fallout and at the first mention of a Fallout 4 I think I just squealed with joy. I've been thinking about this for a bit, here's something I'd like to see in a new Fallout game:
Return to the east coast. I absolutely loved F:3, it was an amazing game, but after I completed it for the twentieth time I couldn't help but feel a bit down knowing that my actions wouldn't link up with the other games, seeing as Fallout 2 was a direct sequel to Fallout, and there were many, many references to these two games in New Vegas. The Capital Wasteland was basically cut off from the other games based in the west. I would love to see a new Fallout based in New York, Savannah, or Boston, as long as its a city on the east coast, I'm happy. However, this doesn't seem likely as Bethesda has given hints about it being set in Chicago, I guess to revive Van Buren, which was supposed to be set in the Midwest? Meh, a game set in Chicago would be great as well.