Most cities of the world do not have a NFL team
The Cowboys
Baltimore went without an NFL team for over 12 years after the Colts left for Indianapolis in 1984. The city regained an NFL franchise in 1996 when the Cleveland Browns relocated and were rebranded as the Baltimore Ravens. During that gap, Baltimore was without professional football at the NFL level until the Ravens' establishment.
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.
Seattle
Miami
Probably NY.
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.