If I had to guess, if the TV show wasn't already, FO5 will be the watershed moment where they move over to the West Coast and finally ruin everything established there.
San Fran was talked about a lot via Kellogg in the same way the Commonwealth was in 3. I also recall there were some mentions of Bethesda telling the NV devs to not use San Fran in case they wanted to make a game there, which why else would you say that unless you had plans to do that?
Plus in the FO3 making of video Todd stated that he wanted to have FO3 set in the WC. If it makes anyone feel better, Emil was the one who pushed for 3 and 4 to be set in the East Coast in the first place, so in a way he saved us all from himself for a time.
Granted a lot of these points are based off of things from 15-17 years ago, but it's food for thought. In all actuality it will likely either be San Fran or God forbid New York. Given Boston, can you imagine how bad the stuttering and frame rate will be?
As for why I say New York, FO4 was actually supposed to take place there originally, and at the end of the game you would have the Institute reveal and would travel by vertibird to Boston to deal with them. It should be noted that they held onto the design documents for this.
I would really prefer if they did some other locales we haven't seen yet, like Ronto, Chicago, and Texas.
None of it really matters, since assuming Bethesda even has it's doors open for that long will take at least another 10 years before we even see FO5 in anyone's steam library.