Most cities of the world do not have a NFL team
The Cowboys
Las Vegas
Pittsburgh Steelers
This has been true for all twenty of the US metropolitan areas with the largest populations; some of which have had TWO NFL and MLB teams at the same time.
Probably NY.
Seattle
Miami
It's the FedEx Field of the Washington Redskins
The question asks for speculation, but the safest bet would be a 1 team expansion to Los Angeles. The NFL gets alot of pressure from the networks for not having a team in the 2nd largest television market, and there has been a variety of plans discussed to address the issue from moving an existing NFL team into the city, or expanding there. This is not imminent or even decided upon.
The largest is Los Angeles, California.
Green Bay is by far the smallest market with an NFL team, with a population of only 101,203.