I think it would be cool if most of the map was the small towns of Massachusetts with a few main coastal cities such as Boston, Newport and Salem.
This would also already provide some structure for storyline of the game. The player could start out in Western Massachussets like in the Springfield area and embark on some sort of quest for whatever reason that leads them to the east coast.
Also the Synth Retention Bureau is located in MIT so it also makes sense to take place in Boston. The SRB and The Railroad could be like the NCR and Legion. Also it would be pretty sweet to fight against/with androids.
"LOCATION: Fallout 4 is set in and around Boston and the surrounding countryside."
"Meanwhile, Vault 79, located outside of Boston, is under the control by "The Railroad", a group of people dedicated to helping Androids escape the institute."
lol my prediction was spot on