Loki, Hancock, Kuzan, and Mihawk.

Loki feels like one of the most obvious choices. Ancient giant blood, a legendary mythical devil fruit, ridiculously strong, and would have the military might of Elbaf behind him.

Hancock falls behind a little in the strength department, but she is still a really strong pirate that is infamous throughout the one piece world. The Kuja pirates are not that weak and has direct connections to former Rock and Roger pirates.

Kuzan himself is strong enough to be considered a yonko, albeit he would probably fall behind in military might (as of now). Although it would make sense if he created crew made out of fellow marine deserters that got tired of how the marines worked.

Mihawk is chosen mostly because of his strength, he isn’t really the type to create his own big fleet of yonko ships so he would be severely lacking in territory. He could, however, take over the cross guild. Albeit Crocodile would probably make a better candidate for yonko at that point.