Indeed, I believed that the legendary fruit was Luffy's since it was mentioned in chapter 1037 but I find that Oda has yet to provide us with further explanations, there are still plotholes
Indeed but I don't believe it was planned since the start. It was planned at least since 2016, when Oda said in an interview that he had yet to figure out how Luffy would take down Kaido.
If you want me to keep going on with the controversial shit, I never believed that Law and Kidd could take down Big Mom.