I like the idea of Big Mom powering up with a Kaido homie to come and fight Luffy on Elbaf, and her wondering where the One Piece is falling down does open the possibility of her coming back, but I just don't think it will happen because Oda pretty much showed everything she might've had in Wano. If she were to return in Elbaf, I don't think she could come back as the main arc antagonist, somebody more along the lines of a callback and shorter fight, like Lucci and Kizaru. If she returns more for callback sake then actual arc antagonist, then I don't think Zoro getting a opponents well would be guaranteed. At best he'd get Smoothie, and fight her similar to the brief clash with Kaku.