Wonder if it could have something to do with Kaido's wish to die. Like, telling him that BB or Luffy (more likely BB, since Shanks had experience of fighting him and getting injured so knows he's powerful, plus he might know the nature of BB's special powers and maybe they can somehow kill Kaido. edit: For Luffy it would be that Shanks guesses that the experience of the war will make him reach new heights) might be the one to kill him one day.