I saw a lot of people talking about how Zoro killing Kaido after Luffy defeats him would be no fun, let me give you my theory:
Do you guys remember back in Zoro's introduction "arc" when Helmeppo took Koby hostage to stop Luffy from hitting Captain Morgan? Luffy just ignored Captain Morgan and wen't after Helmeppo to save Koby, but at that moment Captain Morgan used the distraction to try and kill Luffy, but Luffy just trusted Zoro to finish him off and didn't even look back, that was the only other moment Zoro gave the main antagonist a final blow instead of Luffy.
Now if you remember, the hostage situation was exactly what happened with Oden and his downfall was trying to do all of that alone, caring all the responsability, but we know Luffy isn't like that, he trusts his crew to do what needs to be done.
So Oda could make the same situation again, this time Big Mom could take Law or maybe Robin hostage (she already showed interest and she actually seems to care somewhat about Kaido) and make Luffy stop paying attention to Kaido, and when Luffy goes to save the hostage, Kaido tries to attack Luffy just like he did Oden, but this time our man Zoro aka The Grandmaster, gives a final blow to Kaido so he doesn't interfere with his captain and Pirate King.
This would be an awesome moment and a great callback, Luffy gets to defeat Kaido and Zoro gets to keep his promise and start his legend by killing Kaido / Dragon just like the sword god once did back in Wano.
