The USA. They have a large athletic pool of talent to choose from, and potential investment. All they have to do is get the youth setup sorted out so the players get quality coaching younger, and of course the interest has to wander from other popular sports in the USA.
Really talented players like Messi, Ronaldo come about because they love football at a young age and play everyday out in the street or some street soccer pitch. You need this sort of genius player to win a world cup, unless you play like a machine (germany)
As show in the previous world cup, the US is a force to be reconed with, all they need now is highten the technical quality and a genius player.
I think the US is the most likely country to do it.
Either the US or Japan, I think. The USA basically for the reasons Bicnarok mentioned earlier. Now I'm taking a big risk by saying Japan, but there's been a lot of good young players coming from Japan in recent years (Keisuke Honda, Ryo Miyaichi etc.). I don't think Japan even had a professional league since 1992, so they're making progress very fast.
If neither of these countries would be afraid to hire on foreign staff in the decades to come, both of them could become world champions in the future.