Massachusetts is located to the right of New York, so New York City can also be fit in the game (as a lot of you wanted), and it could have Rhode Island in it, which could have more of the Point Lookout feeling to it (which a lot of you wanted because a lot of you wanted the south to in the next game).
The main factions could be:
The Institute
the Faith
(insert faction for New York here) - Have yet to come up with a good enough faction for this city.
And there could be dozens of little minor factions scattered throughout the Wasteland. - The Commonwealth Wasteland
For those of you who don't know who the Faith are, the location of their HQ is unknown, but since the Missionary from Point Lookout was from the Faith, I would think their HQ could be near the Commonwealth. Since Maryland is close to Massachusetts as well.
If you have any suggestions on what else should be in the game, say it. It'd be perfect for the next Fallout game to be here. =)