I think that if we get ZvK and SvQ and Yamato takes Jack solo then that's a strong hint toward her joining.
Yezzir. I just feel like Oda is going for those clean matchups. Yes, clearly Zoro needs/needed to fight Kaido for the enma plotline, but I dont think it detracts from a main YC fight if they lose on the rooftop. Not only this, but we'll be able to tell if Marco ends up losing or if Sanji fights Queen.
If those 2 things happen, then virtually the only other option is Zoro at that point.