Ace fought Akainu for Whitebeard’s honor and eventually sacrificed his life for his little brother Luffy. Sabo wants to avenge his brother. He knew he couldn’t do it without Ace’s Devil Fruit, so he waited until he got his hands on the Mera Mera no Mi. Now he can learn how to use it and master it to defeat his brother’s killer.
#4-MONKEY D. LUFFY
After defeating Blackbeard and attaining the One Piece, Luffy will declare war on the World Government which Whitebeard predicted and of what the World Government was scared.
In this fight Luffy will fight Akainu to settle the score of the Marineford and defeat him.
*Theory by Samara Jordan