A job not only shares its primary weapon with its class, but also most of its skills. When we consider what additional jobs may spring from a class, we need to look at that class' skillset and ask if those skills make sense for the job in question. So, I really doubt Samurai is going to be a Gladiator offshoot. Samurais don't use shields, and without a shield, how would they use skills like Shield Lob, Shield Bash, Shield Swipe, or Bulwark? On that note, unless they make Red Mage a tank job, I don't think it'd be a Gladiator offshoot either.