What would the advantages be of a single worldwide economy?

If the trades in the world are free,it is called the first best. It will be a win-win situation,and the world as a whole. The gain from trade will be maximized with efficiency.But if it cannot be done that far, a trading block will yield a second best.It is because of trade creation will be more than trade diversion as stated in the economic integration theory.The Doha round if it can be concluded will stimulate the free world trade,including agriculture. But bilateral agreements will also help.