We can't say nobody wants to be the WSS. There were those two Okama during Marineford who challenged him in the past....
And you know what, now that I think about it, that's pretty funny. For as long as the "how close are Zoro and Sanji debate" been around, nobody has realized this little tidbit. The only characters Oda has had come flat out say they challenged Mihawk, where Okamas.
The man really found a way to parallel Zoro and Sanji with Mihawk.