I feel like Luffy would be the one that is looked at but i don't think that their would be a replacement by the end of the series other than Luffy. I just think that people will stop caring about the title of Yonko and more about the hunt for the One Piece. While the worst gen are probs the main contenders that we know about i don't think any of them really want to be a yonko. they want to be the one to find the One Piece.
Id woudnt be imidiate , you would have to prove you are strong enough to be a Yonko.
Same thin with Blackbeard , even after he took WB's DF , he wasnt named Yonko. He was named a Yonko after he captured almost all of WB's teritory. Soo i think that place would be empty untill someone proves he is strong enough.