Well, Ed can always say that after the 40 something years, parts of the city begin to unearth from the sea.
OR: The Founders start building on.
The founders are probably dead after 40 years, they've already been there for many years, meaning they should be atleast 40-50 by now
.. or most of them are already dead
It's just too soon to find out if there are going to be any add ons. But if they were they can create some pretty good DlC's such as clothing addons, What if missions, maps, guns, gun mods. There can be a lot of great DLC's but lets just see if the game goes well.

I'd rather have map DLC than clothing, but if they can add both into one, then whoopidoop, yes!