I would argue that there are 4 major sports leagues in the United States: baseball, basketball, football and hockey. While there are other sports leagues that do exist, they are not as prevalent based on the empirical evidence that they don't appear on my facebook newsfeed, nor do they appear on the sports section of most newspapers.
When I think about the origins of these sports, I think about a time where workers would get off of work, either during a lunch break or at the end of the day, and would throw around a ball or something. Ultimately, these pseudo-sports would catch on among other workers, the rules would become standardized, and a sport would be born.
However, that experience of capitalism is gone. The mass production factory assembly lines are a thing of the past that only resurface when reading Karl Marx or "The Grapes of Wrath." The factories of today are automated, and the workday experience of workers today is lived differently than it used to be more than half a century ago.
Moreover, sociologists have observed that in recent years, people are not attending social events as regularly as they once did 30 years ago. I mean think about it. The bowling league to which Fred Flinstone and Barney Rubble belong represents a antiquated mode of entertainment that no longer reflects the experiences of the common audience (this was an obvious example based on Sociologist Robert Putnam's book "Bowling Alone" where he discusses this phenomenon.)
Today, recreation is more individualized then it has ever been. When we want to have fun, we turn on the computer or TV which are made to be enjoyed in small groups. There are many examples of why this is true. (Consider the TV. Most of the time, when we watch a movie or a TV show, we beg for silence from out peers in order to hear the show and enjoy the plot and all subtle details. while it is true that some TV shows can be watched in groups, those groups are usually pretty small, and not as social as the "tea parties" of the earlier decades.)
Therefore, since entertainment today is more individualized, and since our workplace is less social and more automated, I believe that the potential for new sports to develop is very limited since the opportunities to kill time after a mindless days at work with a larger group of friends, no longer exists.
But I suppose I can't tell the future, and "dead" is such a strong word...