Obviously it has to be a mythical since BB only goes for the best. I used to think he would kill Kaido and take his Zoan since Kaido has a good claim to having the strongest Zoan, but I don’t know if Oda would want to be repetitive and have Luffy fight another Dragon DF user, and there hasn’t been any setup to BB coming to Wano to finish off Kaido. I think a Kraken DF would make more sense. It fits BB’s characterization better since he is one of the most realistic pirates in the series. It’d be cool to see BB transform into a Cthulhu-looking hybrid while also using his Yami and Gura powers.