Readmanga – Before Luffy become pirate king he will beat both Akainu and Blackbeard and by doing that he will get his revenge for Ace’s death. He will fight Akainu first, beat him and then continue to Raftel(i can also see Sabo beating Akainu). But before he gets to the island he meets Blackbeard and fight him.
After a long fight Luffy then arrives at Raftel and become pirate king. Because Luffy(or Sabo) had beaten Akainu, the marines will need a new fleet admiral, and i thought about Coby. Think about that, he was there since the beginning and he wanted to be an admiral, so maybe he will be an admiral before Akainu gets beaten and then become the fleet admiral after Akainu is beaten. Now you probably say “Coby? but he is weak” and you’d be right. Coby is weak, for now, but that dosen’t mean he won’t become stronger until the end of the series, especially if he gets a devil fruit. Plus Luffy and Coby could have what roger and Garp had.
Coby could chase after Luffy and will fight him multiple times just like Garp and Roger did, it will be a closure.