Two answers allowed
1-Someone that Luffy already met but looong time ago (like Shanks, Dragon, Sabo for example)
2-Someone that has been mentioned but Luffy never met (like Vegapunk, Scopper Gabban, Kaido for example)
1-Dragon. Because I suppose that someone of the contender in the Colosseum is a member of the Revolutionary Army
2-Big Mom. I think that Big Mom will mess up Law plans against Kaido in some way.