personally, I'd like to see the following guys challenge Rhino...
8 sided ring - Ken Shamrock, Tito Ortiz, Royce Gracie, Chuck Liddel
6 Sided - Joe, Hardy, Raven, or Sting
4 Sided - Bryan Danielson, New Jack, Sabu, Sandman
but we all know WWE wont let Sandman or Sabu take on Rhino