What will be the motivation behind this fight? What will be the reason?
Luffy doesn't want to destroy the marines. He fights marines when its necessary, he usually has good relationship with the marines; Coby, Aokiji, Fujitora etc. You can say even with Smoker. Fighting the Fleet Admiral for what reason? If a war happens like the last one to save a person then maybe, but then is Fleet Admiral going to fight at the beginning? Sengoku didn't fight until it was necessary, I doubt Akainu will. So its really look like nothing but fan wishes so far. If Luffy saves the person he wanted to save then they escape. If that person die again, Luffy fails there is no point on beating Akainu for revenge.
I think at some point there will be a war close to marijoa and in that war Akainu will fail and they replace Akainu with another Fleet Admiral.
I think this will happen before the final war against the government, if it happens.
Because government has their own forces other than the marines, remember what Gorousei said to Akainu, the Marines are the just ''outward face'' of the government. The final war against the government, if happens, will happen against the ''real face'' of the government which is not the marines.
That's also why Kong commands the other government forces that we don't know yet.
CP0, Kong, Vegapunk, and Imu there are many other unkown forces for them to deal with, the Marines can't be the main force in the final war against the government. Side force maybe, but then Akainu might not be the Fleet Admiral at the time. Or the marines might not join in the war which would be better since the marines should be the good guys and they shouldn't be destroyed by a pirate. The evil government yes, but not the marines.
Gorousei were already not happy with the departure of Kuzan and him working with Blackbeard and I doubt they liked what Fujitora did in Dressrosa as well. If SWORD is a secret organisation that Akainu doesn't know then that will be even a bigger problem for Akainu, meaning that some marines are serving to someone else not to Akainu.
Remember Akainu and others didn't know G5 marine base commander vice-admiral Vergo being Doflamingo's spy, SWORD could be unkown to Akainu like this. And there could be other spies like Vergo inside the marines that Akainu still doesn't know. That could lead Akainu to another failure and the government and Kong could decide to replace Akainu with another Admiral.