They are more likely to one day be middle-weights at rugby union than football.
Give you an example or what I mean using my own country;
Australia sits in ranking between 14 and 30 (most of the time).
Just the same that is using the best players we have available.
There are nations in the top ten who could field four or five teams the equivalent of nations like Australia. They could field even more so teams the equivalent of nations further down the ladder.
With this in mind I doubt the US will even be a big name in football, however they are spending a fortune on Rugby Union, are likely to host the rugby world cup in 2023, and now have an intercollegiate competition as well.
Not saying it will ever be a threat to established sports in the US, so don't jump up and down, just saying that of all the sports played around the world that are not widely played in the states, the one most likely to grow is Rugby Union.