I can see arguments for Imu vs Dragon, or Imu vs Sabo, or Imu vs Sabo + Dragon, but also they work for Imu vs Luffy, or Imu vs Luffy vs Blackbeard (ripped wanted posters).
But Akainu fits nowhere as a final villain for Luffy. Neither is he a D, neither a CD, zero ties with the ancient past, Imu, BB have all of them, he either gets beaten earlier by Luffy, or becomes an opponent for Sabo at the finale.