Excellent question. I see a lot of people with this doubt, after all, if Kiado is the strongest, what's the tension of fighting someone weaker later?
But I answer you. First let's remember that Oda declared that One Piece should end in another 5 years. As everyone knows, it will last longer, but the meaning that matters to us is that this is the final stretch.
Wano started in 2018, even if One Piece spans another 10 years, it's possible that Oda will only make 2 more arcs.
What I'm saying with all this is that Luffy won't have many enemies anymore, probably just Teach and Im and here we answer his question.
I don't think Luffy will fight Akainu or some other antagonist on Elbaf. As a matter of time and narrative too, I only see him fighting with Teach who, when he gets the third fruit, will surpass Kaido. And in the end we have Im because no one knows of his existence I believe it is valid to think that he can be stronger than Kaido.