I used to think just like everyone else, that Luffy and Akainu were going to duke it out over Ace's death. However, it seems to me that having two immensely powerful FVs will give Luffy way too much credit. Blackbeard and Akainu both are forces to be reckoned with, so I don't see Luffy pounding down both of them.
That's where Sabo comes in. Sabo was getting stronger behind the scenes for a long time, and in front of us readers, experienced development of his skills by eating the MM. I'm thinking that this is a taste of what's to come; Sabo will continue getting more powerful as the story progresses and he will be the one to avenge Ace. He even has Ace's old powers now. Luffy may have seen Ace die, but Sabo is no less entitled to revenge for his brother's death.
That's where Sabo comes in. Sabo was getting stronger behind the scenes for a long time, and in front of us readers, experienced development of his skills by eating the MM. I'm thinking that this is a taste of what's to come; Sabo will continue getting more powerful as the story progresses and he will be the one to avenge Ace. He even has Ace's old powers now. Luffy may have seen Ace die, but Sabo is no less entitled to revenge for his brother's death.