@Dan:
Not sure why so many people seem to be so heavily into the 'Barto is friend' camp. I mean maybe, sure, but it doesn't seem especially likely at this point.
For me, because it feels like it was phrased so ambiguously. When there's something like that, I go with the interpretation that the author has a reason for hiding. The scene is framed as rather sinister, Bart has been presented as almost comically terrible, there are two other characters who unambiguously stated their intentions to kill Luffy - but for some reason Bart doesn't just state outright that he wants to fight him. The conversation feels like a classic misdirect. Bart says something that can really be taken in the opposite direction (not wanting Luffy to be hurt at all), only for Cavendish to jump to his own conclusions. And Cavendish is exactly the sort of character who would just project his own feelings on someone else and not consider the other way of reading that sentence. Maybe it is straightforward (Bart's a bad guy, yet another competitor that has a grudge against Luffy), and any ambiguity was unintentional. But for now, that way this scene is set out ("I won't let you kill him", character who doesn't know him makes assumptions) makes me think that Bart likes Luffy.
Luffy's saved so many people that one of them becoming a huge fan and wanting to repay some debt is definitely feasible (just look at the kiddos in Punk Hazard wanting to be pirates.) Question is who - Bart doesn't look like he'd be from Nami's home island or Alabasta or anywhere we've seen really. If Bart's connection was with Ace (based on the name theory), I think he's too new to have been a Spade pirate or a peer, since he just came on the scene a year ago. But Ace could have saved him or inspired him. Ace liked to talk a lot about Luffy, according to Jinbe and Marco, so I don't think it would be a stretch to call that a deeper connection.