No current franchise with one film can be compared to an 8 film franchise like Harry Potter, but Avatar is the highest grossing film in history, both domestically and internationally. Give the franchise another film or two and it could easily compete with Harry Potter.
It would also need 7 books that were on the NY TIMES Bestsellers list, a loyal fan following spanning various age groups ,toy/game tie ins etc...Avatar has its fans, add a movie or 2 more and it will have more...But I don't think it will come close the appeal of Harry Potter.Ask any small kid and they can tell you who Harry Potter is( and any adult for that matter) ..I don't see the same thing happening for Avatar( though James Cameron wants you to believe differently)