I agree this will be a good battle and arc overall.
I'm a huge Akainu fan, but I always thought that he is not suitable for the final villain, as well as BB. For a long time, I was thinking that Gorosei would be the final villains, but the problem with Gorosei was the fact that all 5 of them were equal. But after Imu's introduction, it became obvious to me that Imu will be the final villain. As for Akainu, I think he will stand as the main Imu's subordinate during the Final War. Akainu is a fanatical soldier who wants to purge evil. And like in WOW, Scarlet Crusade was corrupted and led by a Demon, I believe half of the Marrine organization will be on Imu's side, and will even get demonic powers from him. Would be really interesting if Hibari is indeed Akaunu's daughter, or if Akainu was the one who created the Sword organization, which in the end will fight against Imu and be loyal to WG Marines.
Akainu can be connected to the endgame prophecy. In Ragnarok Mythology, one of the final "bosses" was Surtr, the Magma Giant. I believe Akainu will become Hell Dog Surtr after getting buffed by Imu.