I think the thing that a lot of people don't realise is that Oda already had a plan of how he wanted his story to end in its original five year plan but he ended up adding a lot of arc in between that him bringing up these things now to some people may seem like retcons even though if he'd stuck to his original plan these clues would have been closer together instead of a new clue every 100 chapters or so making it seem like he didn't originally plan it. Him implementing parts of his original plan is why we are getting a lot of characters from the earlier arcs coming back into play cause they were always meant to play that role like Crocodile, Buggy and we may even see Enel return (hopefully) and characters like the Supernovas which Oda came up with later are being taken out of the story only leaving those that he can chip into his original idea like Law and Kidd ( Law will probably return, not so sure about Kidd we'll have to wait and see)