Given the political climate in the US, hard to say where the USD will be in 4 years. In any case, I think $3 billion WW is my optimistic prediction for now. If China explodes, I can see Avatar 2 beating the $2 billion OS record of Avatar even if exchange rates are still worse. The question is whether the sequels will see large drops like Star Wars films or will Avatar 5 end up being the biggest of the franchise.