4. Juan Manuel Marquez
A multi-weight world champion and a legend of the sport, the Mexican hero has been in with the best of them. A master counter-puncher who has got more powerful with age, Marquez would give Brook plenty to think about and would be a huge name should Kell get past him. He has not fought since his points win over Mike Alvarado in May 2014 but has been linked with Brook in the past.