I think an East Coast city definitely makes the most sense. Realistically though, they probably can't get enough land in the ideal spot of NYC/Jersey meaning the next best options are DC, Philly and Atlanta. Baltimore could be a dark horse candidate. There's already a lot of construction going on there courtesy of Under Armour building out a new HQ. Charlotte could be an option as well.
My money would be on D.C. Lots of access to government agencies that could add a lot of AWS business, a major concentration of highly-skilled tech workers and colleges, close to the Washington Post, and a lot of up and coming neighborhoods in the southwest/southeast quadrants of the city. Also wouldn't hurt that it's on the opposite side of the country to add some geographic diversity.
As someone who lives in DC: I'm with you on the DC metro making some sense, but DC itself is probably out. There's not enough green space to build a campus the size of their Seattle HQ anywhere in the city, given how the severe building height limits here would balloon the total required acreage. They might have some options someplace in the suburbs (maybe Arlington, or out along the Silver Line), or they could look at Baltimore: still pretty close to DC, but much cheaper, and with a lot more industrial land ripe for repurposing.
Building a 50,000 person campus in the Boston area would be challenging and expensive. Areas that have the space are very, very pricey, and they're all outside the city. Public transportation is on the poorer side, and once you're out in suburbia many streets are too narrow to comfortably fit a bus, so the logistics just get harder.
Baltimore would be a great choice. Easy to draw talent from DC area -- housing prices much lower. Plenty of land both right in town (sad, actually) and in the suburbs. Would be a very big fish in town, and could do much to revitalize an entire city. World-class university in-town. Has a subway. DC right next door to lobby like crazy. And don't forget that Bezos has purchased a house in DC himself.
