The NFL has not expanded since 2002, but that could change soon.

The league has boomed to a $15 billion annual business. Even so, the NFL has watched its popularity flatten out and, in some cases, shrink, as rising cable costs reduces television viewership, the risk of injury via concussion diminishes participation and social justice protests rub fans the wrong way.

How can the league capitalize on the relative prosperity of its product and continue to grow the sport of football? Expansion.

If and when the NFL expands — to as many as 40 teams — here are the places it should look.