American football is a real niche sport much more so than ice hockey or basketball.
You want to make soccer an exciting sport? Allow free substitutions, even in the middle of play. This is the one thing that separated hockey from soccer once they made this single rule change back in the early part of the 20th century. Hockey used to be very similar to soccer more than 100 years ago, but this one change revolutionized the game and indirectly gave it some of its distinguishing characteristics. For one thing, it made hockey a transition game rather than a possession game, and made it an anaerobic activity marked by short bursts of frenetic energy. It also separated itself from other major sports by getting every player on the roster involved in a game, thereby adding a value to playing while injured ... which is where the game's "tough-guy" roots began.