While the NFL by and large continues to grow year after year, the biggest hurdle that the league ultimately faces in terms of expansion is the fact that football has always remained a nationally regional sport, rather than an internationally-viable game such as soccer.
The league pays lip service to the idea of expanding its reach, committing to playing games elsewhere, primarily in England, but more recently has committed to playing a certain number of games in other countries, such as in Mexico, where there is a largely healthy fanbase for the sport.
The Pittsburgh Steelers are far and away among the most popular NFL teams in the country, insofar as any team has national appeal, but the team has talked about potentially playing a game there without it ever developing into a concrete plan.