Philadelphia is perfect for this. It's got it's own burgeoning tech scene, but it specializes in education and medicine for the most part, due to the lack of large tech firms in the area.
It's got 1.5 million people, meeting the larger than 1m population requirement, and a metro area of over 4 million.
It's centrally located in an area that it is very easy to attact talent. It's less than 3 hours from New York City, Baltimore, and Washington, DC. 5 hours from Boston and Pittsburgh.
It's also got the infrastructure. The city was developed to house and maintain living for 2 million residents, but the rust belt decline hit it hard and the population is now only 1.5 million.
It's on the East coast, which is probably the ideal location for doing business with Europe, and would be opposite the country of their original Seattle offices.
It's got a massive plot of land at the Navy Yard which is currently housing many exciting companies and is a massive economic driver, but it's also a Keystone Opportunity Zone, which means it's abated from nearly every Pennsylvania State Tax and Philadelphia City Tax, which is obviously going to be interesting to Amazon.
It's got a major international airport, a major international port, and sits on one of the greatest shipping hubs in the USA. Philadelphia is the 7th largest shipping hub, and it's an hour and a half from NYC/NJ which is the 6th largest.
It's also got more art, culture, and history than almost any other American city.
Seems perfect Amazon, you know what to do. We even have the eastcoast flagship Whole Foods store, and you guys just bought them.