I get the feeling that most WBC teams want to take out the USA. They are largely considered the best and its always fun to beat the best. But when it becomes, lets beat the best by using some of the bests players. To my knowledge in soccer, once you declare to one country you are bound to play for that county's national team in FIFA events. For example ARod, once he plays in the first one for the USA he is bound to play each subsequent one for the USA.
For competitions sake I think anyone who follows the WBC would like to see a Final 4 of USA, Japan, Dominican Republic then either a Puerto Rico or Venezuela. I would do my best to make the teams match up so you got that level of competition in the later rounds.