I'm guessing, since Luffy calmly went along with it, that it wasn't Shanks. It'd be cool if it was Kaido, since we haven't seen or heard much from him, though I'd also expect it to be Blackbeard, or perhaps even Big Mom.
I think it's Kaido, ALTHOUGH Shanks took care of him 2 years ago, but that's nothing to say it isn't him. If it was Blackbeard, I have a feeling that Luffy would of at least put a serious face or if it was Big Mom, he may of said to Law ''I've already declared war with her, so why not.''
This is why I'm baffled xD Unless there's a new Emperor that we don't know about.