Although we have no background for that I believe Katakuri is the one to be Yonko Level when BM is gone for whatever reason.
Without timeskip the only character I remember to get a lot stronger was Crocodile. Besides him nobody. Buggy didn't. Smoker didn't. Moria didn't.
After timeskip we have some people coming back stronger of course, Bellamy, Lucci etc.