I'm sure we'd see a confrontation between Akainu & Dragon at one point, due to implied past history or atleast what I assume is implied past history. For me I think Fujitora's decision is gonna determine who Akainu fights, if Dragon gets Fujitora to turn on the World Government (which is also why I'm interested heavily in the RA commanders vs the 2 admirals, since I don't think it's a mere coincidence that the two admirals the RA commanders fought were the anti-WG admirals), if that happens then I see Fujitora taking on Akainu. You then the iconic team up of Dragon & Tiger, symbolically the two symbols of power in the japanese & chinese mythos. If Fujitora remains the same then yea between Luffy or Sabo for who fights Akainu.
Even if we take into account Akainu embodies absolute justice, fact is though he's not the leader of the entire World Government Military in it's entirety, it's also why I still have my reserves about Akainu even being the big baddie fight on the World Government side even before Imu. Because you have a guy who was running the Marines from Roger/Rocks era and now running the WG military since WB's era, while everyone else is either dead or semi-retired, this mf still running shit.