Puerto Rico voted to become a state, though there is some dispute over the results. They're not a state yet because their request would have to pass Puerto Rico's own legal hurdles and then be ratified by Congress. There's also some chance Guam or Samoa could get statehood, slim though those possibilities are. Maybe Alberta will get p.o.ed at Canada and request to join. (j/k). In any event, there is some chance we'll need a 51-star flag at some point. Here are two possible designs. I'll name them above each so that you can vote for whichever one you like best.
Basic Design (essentially the same flag with just one more star)
Circular Stars (stars forming a circle for the first time since we had only 13 states)
Of course another option would be to make both flags official. You would have the choice of which one you want to fly.