6. Spartacus
As we all know, thanks to Kirk Douglas and Tony Curtis, Spartacus was an ultra badass former slave who dared to take on the entire Roman Empire. Just the kind of odds the Doctor likes. Spartacus was a former gladiator who led a slave uprising against one of the most powerful enemies in all of history, and there are no are reliable records of what happened to him after the war. Although some historians have suggested that Spartacus died during one of the last battles of the Third Servile War, reports are dubious and no body was ever found. I can see the Doctor having an interesting, yet tentative, relationship with Spartacus. He would sympathize with the desire to rise up against oppressors, but would be very uncomfortable with the level of violence involved. I think it would be really cool for Spartacus to somehow get pulled out of time, and for the Doctor to have an adventure with the gladiator in some other time period. You would have the fish out of water element, with the Doctor desperately trying to curb his more violent tendencies.