A reddit user by the name of ‘fallout4boston‘ claims that the next title in the series is going to be set in Boston.
If the rumor is true then Bethesda is already researching the city for it’s unannounced game. On being asked if the rumor was true or not Bethesda replied that they do not comment on rumors.
The reddit tipster further said Bethesda has strong connections with the Massachusetts Institute of Technology, a prestigious research university near Boston. Both Fallout 3 and Fallou: New Vegas mention a mysterious institute located in the commonwealth. This could probably be related to the Massachusetts Institute of Technology.
The last Fallout was released back in 2010 and was based in a postapocalyptic Las Vegas. The one before that was based in Washington DC.