I've talked to some soccer people, and they've told me athleticism isn't what holds the U.S. national team or American players trying to make it in Europe back. If anything, in most international tournaments, the Americans are going to be some of the fastest, strongest, and most athletic squads on the pitch.
The issue is skill and experience. Europe, Latin American countries, and the rest of the world just have more kids getting more touches from an earlier age, which turns into a cumulative benefit by the time they are young adults and ready to start playing competitively. It's a "cultural" thing we simply lack.
Plus, they told me that bringing in the elite athletes from more traditionally American sports (football, basketball, etc.) wouldn't really help. Soccer is about skill but it's also a form of distance running, and the height, strength, and bulk you need to play "American" sports can be more of a hindrance there than help.
There's a reason Lionel Messi is 5'7" and 160 pounds. He (and the other players of the same slight build as him) are obviously in great shape and incredibly strong for their size, but they are still small because that is the most efficient way to play 90 minutes. Extra height and bulk would just drag you down.