The Endgame for the One Piece begins, with all parties going into battle. There won't be a fight with the WG "after the fight for the One Piece has been decided". The WG won't wait for such a main event to occur, since getting the One Piece and becoming the PK will be the WG's doom. Roger himself already shook the world, while saying "they were too early", implying Luffy's generation will be just on point.
With either Blackbeard or/and Im being set as the final villains, Akainu will very likely be set up fighting Sabo. While Sabo wants to go against the Celestials, he has no one to fight there, because the Celestials are fodder and Im is obviously out of Sabo's scope. And since the admirals are the Celestials greatest shields, fighting Akainu would be fitting, especially due to the dynamics they have with Ace.
It's no shame for Akainu to lose to Sabo though. Sabo will be a power house by that time.