I think the best places for the next fallout are 1) Buffalo/Niagara area or 2) the South East (preferably near the Mississippi). This is for the following reasons.
One I do not think the next game should be anywhere near where previous games took place. Enough time should pass in those areas so before making a canon ending so not to make a persons choices in the game redundant. I mean what is the point of doing whatever you want if the next game chooses a cannon ending.
Two these are areas that havent been done so this could be a area where obsidian can make a game that does not interfere with the story made by Bethesda.
Buffalo would be a good place because it is next to the commonwealth and Niagara falls. Also it could include Toronto, so people that want to see a place outside the US would be happy. Toronto was hinted as a military power in the pitt. a confrontation between the commonwealth with the synthoids and Ronto would be cool.
The south would be the best though. Cape Hope was a great DLC and I would love to have a game similar to that. Plus if it is next to the Mississippi we could have a DLC anywhere on the Mississippi. Or we could have a DLC about the only diamond mine in the US in Arkansas. Blood diamonds in the US. A international touch in the US that could appeal to people. Or New Orleans, etc. There are many places in the south that we would enjoy.