I'm gonna have to say that Lacrosse will be the next sport to tear across America. Lacrosse already has a minor foothold in some areas of the country, and there are a lot of things that can contribute to its expansion. First, Lacrosse was developed by Native Americans, so in this way it is a historically American pastime, and I think a lot of people will take pride in that. There's something about knowing people in your area have been playing a sport for hundreds of years that makes it more enjoyable.
Secondly, Lacrosse is very reflective of life in America - fast-paced and exciting. As opposed to a slower sport such as baseball, Lacrosse will keep your pulse pounding non-stop, whether you are playing or just watching. Lacrosse features a lot of scoring and dazzling plays, most of which would make SportCenter's Top 10 on a daily basis. This is also a contact sport, which makes it ideal for the American Alpha Male to play and exciting for anyone else to watch.
Lacrosse combines the finesse play and toughness that you see in hockey, except it can be played almost anywhere, in pretty much any type of conditions, which should help it gain a larger following. The rules of the game are also pretty easy to understand, so there isn't a big learning curve or a lot of complex conditions that will leave people wondering what is going on. All Lacrosse really needs at this point is some good marketing, a few big names and a couple of major markets, and it will be the next big American sport.