Assuming the US government actually did allow this, which routes would foreign carriers want to compete on? Personally, I don't really see foreign carriers flying many more routes than the ones that already have a lot of competition (e.g. LAX-JFK/EWR). Even tag-on routes (like the soon to be former AKL-LAX-LHR route or Quantas' SYD-LAX-JFK) might become less important as more efficient aircraft that can do long direct hops come online.