[Blackbeard D. Kuma]

And as mentioned already, the yonkou in question doesn't have to be taken down in the immediate future. This alliance may last for quite a long time, after all. And if people are uncomfortable with the idea of Law tagging along with the SHs for a good chunk of the story, it doesn't necessarily have to be that way. They could always split ways temporarily and regroup at later points in the story. That being said, Shanks is within the realm of possibility. And it's so like Oda to want to make us think that Shanks is least likely to be the target, but we all know Oda is renowned for implementing twists into his work.