We all know that Coby and Sakazuki have different ideals and have different opinions on Justice. During their first coming together, Sakazuki almost killed Coby if it wasn’t for Shanks. Coby is a mixture of both Garp and Sengoku. His goal is to become an Admiral; he has also mentioned that he wants to become the ultimate Marine which may lead to him becoming the Fleet Admiral.
Coby as we know is under the tutelage of Garp and has learnt to fight in a similar fashion, using Haki, Rokushiki and brute strength.
However, as mentioned he has similarities with Sengoku which means that he’ll eventually eat a devil fruit and that devil fruit is gonna be the Mera Mera no Mi. There have been many rumours regarding Sabo’s death at Reverie by the hands of Kizaru. The Crab can be symbolised through Sabo as he has claws. He fights with claws as a crab does with its pincers.
At the time of Sabo’s death, Coby will be present and he will inherit Ace’s will. Hence, he will be the new “Firefist”. Now we reach the conclusion of this theory.