According to an interview with Eiichiro Oda, he stated that if Akainu were to become the protagonist, he is so strong that he could end the One Piece series within a year.
Akainu possess an immense amount of superhuman durability (as not even Marco, Vista, and Jinbe, who are highly skilled Busoushoku Haki users, could not deal any significant damage to him) and a tremendous strength.
He ate the Magu Magu no Mi, a Logia-class Devil Fruit which grants him the ability to transform his body into magma and manipulate it at will. As a Logia, Sakazuki can let physical attacks pass through him without injury and can only be wounded with the use of Haki, Seastone, being caught off guard and his elemental weakness.
Akainu’s ability is naturally of a higher order than Ace’s pyrokinetic abilities, as the higher heat and smothering abilities of his magma allowed him to successfully harm Ace despite Ace’s Logia-defense, and ultimately kill Ace due to this superiority.
The destructive heat and explosive powers from the magma enables Akainu to cause massive damage, and to take on people like Emporio Ivankov and Jinbe with minimal effort.
During the decisive ten day battle on Punk Hazard for the position of fleet admiral, Akainu showed the furious power of his Devil Fruit as he changed the weather of one side of Punk Hazard and left it in a blaze. Because of this, many fans speculate Akainu has awakened his Devil Fruit.
It was stated that all vice admirals possess Haki, so being two ranks above vice admiral, Akainu can also use this ability. He was seen using his Busoshoku Haki to negate Whitebeard’s attempt to destroy the execution platform, by creating, with his fellow admirals, a shield combination of their three Haki.
It’s very likely that Akainu is going to be shown as almost equal or maybe even above the power of a few of the Yonkos.