This is how I believe the future fight against Big Mom will go. Before completely losing his consciousness, Katakuri asked Luffy if he will return to the Whole Cake Island to defeat Big Mom. Without hesitation, Luffy answered that he would come back and keep his word since he’s the person who will become the Pirate King.
So I don’t think this is the end of Big Mom’s role in the story. I don’t think Big Mom will die from hunger or a Buster Call, I believe Luffy will take down all of the Emperors with the exception of Shanks.
So what better way for the two to fight than in Elbaf, the place that Big Mom has so much history with and the place the story has foreshadowed since Little Garden.
But why? Why would Big Mom go to Elbaf in the first place?
Well, there is a missing Road Poneglyph in the world of One Piece. Many believe it to be in Mariejois or in Wano but I believe it to be in Elbaf. So Big Mom, finding out this information somehow, travels to Elbaf to get the final missing Road Poneglyph but Luffy meets her there for a final fight. Think about it, we see a young Hajrudin mad about the Giants letting Big Mom live, the Giants all hate her, and Elbaf is deep in the New World. It would be a perfect place for a climatic showdown with an Emperor. Right now, Luffy is barely at Yonko Commander level but after Wano and Kaido, I can see him gaining several significant power boosts. Maybe in Elbaf, he has a rematch with Katakuri and beats him much easier, proving that he is ready for Big Mom herself.
*Theory by JBAG321