User blog comment:Blackendedsoul/Simple question in a crooked manner, Yonkou selection./@comment-3242083-20110621145113

Probably simply by taking the territory of other pirates, at least this seems to be what the current Yonkou do.

So I guess they are not nominated by anyone, they simple earn the rank automatically after overtaking a certain number of islands and nations. That said, I'm not sure if this is what Blackbeard is after, even if he undoubtedly has the strength and power of a Yonkou.

Kidd or X Drake could be candidates...since the former has the perfect ability to overthrow a nation and the latter already began attacking Yonkou territory (tho I don't know what his real motive was)

But one thing is kinda certain, Marco could not replace Whitebeard, because if he did fishman island would be not under Big mams control/protection.