So, looking back at Fallout 3, and looking at where Fallout 4 is set, I have come to notice sort of a pattern.

Looking at Fallout 3

-It takes place in D.C., which contains part of Maryland, Virginia, and West Virginia.
-In Fallout 3 we heard of Great Lanta, presumably Atlantic City in New Jersey.
-We also hear of Philly, in that people live there, but its been reduced to "that dump".
-We also hear of The Pitt, the ruins of Pittsburgh.
-In The Pitt DLC, we get to visit the Pitt, the largest city in post war Pennsylvania, and we also get to hear of The Erie Stretch, which is presumably around the town of Erie in Pennsylvania.

This means that Fallout 3 either directly, or indirectly, covered what Bethesda decided to make the big citites/locations in Maryland, Virginia, West Virginia, New Jersey, and Pennsylvania.

Now looking at Fallout 4
-Fallout 4 takes place in Boston Mass.
-Boston has easy access to several other states, such as Vermont, New Hampshire, Maine, Connecticut, and Rhode Island.
-Most of these states are very small, both in total land area and population wise.
-Most of these states also lack major cities, at least compared to places like Pittsburgh, D.C., Boston, New York, etc. etc.
-The only other states next to Mass. is New York
-We know that The Abbey of the Road exists someplace west of The Commonwealth, which would put it in eastern New York.

While Fallut 4 isnt out yet, I am willing to bet that the states around Mass. such as Vermont, New Hampshire, Maine, Connecticut, and Rhode Island, will get brief mentions of places, like Great Lanta in Fallout 3. New York will get its own version of The Pitt DLC, via The Abbey, and that will mention a few other places in New york, such as the Eire Stretch was in The Pitt.

This will mean Bethesda would have effectively covered the entire north eastern part of America, and with Fallout 3 included, they would have covered everything down to Virginia.

Now, looking forward into the future, for Fallout 5, where would they go?

Well, in Fallout 3 we heard of The Broken/Crater Banks, a place south of D.C., in what is presumably the rmeains of North Carolina's Outer Banks. If Bethesda keep on their track record of having games cover multiple states in lore, I am thinking that Fallout 5 will take place in some city in the Carolinas, or Georgia, and cover all three of those states in lore, and feature The Broken Banks as a Pitt style DLC.

If they do this, it will mean they would have covered everything from Maine down to Georgia, the entire east coast, forming a sort of "East Coast trilogy".

OFC this is all speculatory, and is largely based on what Bethesda does with the states surrounding Mass. in Fallout 4, but it is interesting to think about IMO.