Coby, Smoker or Kuzan. Luffy killing Akainu would do nothing else but satisfy his thirst for vengeance (Luffy doesn't do revenge). He has no reason to even fight Akainu.
1. Smoker fights Akainu in the promised war and kills him/beats him. Thus becoming the new Fleet Admiral.
2. Kuzan does a rematch with Akainu in the promised war (now allied with the revs and Luffy) he beats Akainu and becomes the next Fleet Admiral (the way Sengoku wanted it to be)
3. Akainu tries to kill Coby due to "betrayal" Garp ends up saving Coby's life but in exchange of his own. Coby gets enraged when he finds out about this and challenges Akainu in the promised war. He beats him.
I think #1 is most likely to happen. Considering that Kuzan never even wanted to become Fleet Admiral and i don't see Akainu killing Garp.
The way i see it. Dragon will take on a much more powerful enemy than Akainu. Maybe Kong or the Gorosei.