I honestly don't see what value Akainu can add as a future opponent to Luffy. Kaido turned into a humongous magma-like fire dragon covering the entire sky in an island-wide, dense fire-carpet able to melt everything away. And that is just the cream topping of Kaido's ability, who has it all; being an Oni with a Dragon Fruit which gives him humongous durability, endurance and recovery, along with high physical strength. On top of that his Hakis near maxed out; FS, barrier Haki, high level CoC coating and extremely devastating blows with his club.
What is the point of having Akainu as a villain after Kaido?