Yes, Lucci was a main opponent somewhere in the middle of pre TS, just as Luffy was fighting Shichibukai and commanders. But now Luffy is near peak; he is Joyboy returned and ready to liberate the world, and at the top of the WG stands an entity who the Gorosei bow down to as a deity. We are talking about endgame here, how can this be bestowed upon a tool that falls to command? Sure, Akainu is not afraid of barking at his superiors, just as Aokiji decided to leave the navy and join hands with Blackbeard, but I don't really see the great implication here, just because Akainu is an executioner of the WG's will. To abolish the WG and the order, the root cause has to be fought, not just one of its symptoms.
Hence imo Akainu fighting Dragon (maybe in addition to Sabo, maybe solo) would make more sense, since Dragon initially also wanted to go against the WG and abolish it, but with him realizing that the Imu/Joyboy lore is way out of his scope, with him consequently fighting the WG's outer face (Akainu) who wants to push his absolute Justice forward on behalf of the WG without himself understanding the grander picture.