In the last chapter of the manga an unnamed character from Roger ship has taken my attention. I am talking about this man:
We know that his age is around the same that Buggy and Shanks thanks to Buggy flashback near to the beginning of the series:
It is possible that Roger had various apprentices in his ship, the most curious thing is that Oda has shown this unnamed character 2 times and if we add that he is at the same age than Shanks and Buggy then if he continues being a pirate he must be in his prime now.
Now, let´s return to the famous Yonkou picture:
If you see the form of the face of the unnamed yonkou that is in the bottom left and our unnamed "apprentice" in Roger ship then we have an huge possibility that he is Kaidou. I know that some people think that I am wrong but the fact that Oda has shown this unnamed character 2 times in the manga makes me think this way.