The notion of Pirate King is often misunderstood. If you watch episode 400 then luffy answers the question of what it means to be pirate king. Pirate King according to Luffy "has the most freedom in the whole ocean". He doesn't want to conquer the grand line or rule over pirates or government. He just wants to enjoy his life and savour the adventure. Rayleigh adds that people created the Great Pirate Age and very few people actually understood what Roger intended to say that day. Also it was useless to give the title of 'Pirate King' to a person who was dying. It is true that opening lines credit Gold Roger as Pirate King who had wealth, fame & power yet whitebeard who must have equal or more wealth, fame and power is not none as pirate king. Also Luffy is not interested in wealth, fame or power neither was Roger since he rejected shiki's proposal to join with him and rule the world. So I believe one who completes the full round of world should be awarded the tag of Pirate King and luffy intends to do that.