[Blackbeard D. Kuma]

The Yonkou are the top 4 pirates indigenous to the New World. It's only right that at least one of them should be a villain, otherwise the strawhats will have things too favorable throughout their travels in the New World. This is all the more reason why Blackbeard should be the one to fill the void left behind by Whitebeard. The strawhats already have 2 emperor level forces on their side (Red-Hair Pirates and the remnants of the Whitebeard Pirates); and if Big Mom really is Lola's mother, that's another emperor ally for the strawhats. Having 2 enemy emperors in Blackbeard and Kaido would balance things out.