I'm not a big fan of Rebecca at all. Never really have been. I like her character, but I don't feel any significant weight to her as a character at all. She's got a sad past, but beyond that she's got no qualities that would make her worthy as a full time Strawhat. She'd never abandon her country and father (who she just remembered she had) to go and play Pirate, especially when it's going to have to be rebuilt after all this is over. The reverse is true with Kyros as well. While this is similar to the situation with Vivi, in which she couldn't go because she had obligations towards country, the difference is I don't even think Luffy is considering or will even ask Rebecca to join, whereas Vivi is pretty much an honorary Strawhat at this point.
She has the potential to change. Even though she's already been hinted at joining the crew, Oda needs to change 2 things so she'd be perfect.
Actually Rebecca has no ties to her country. She doesn't give 2 fucks about it. She may be of the Royal bloodline but she's never been committed to the country or it's people like Vivi was. All she cares about is Kyros. She never lived as a princess nor does she know the first thing about being a princess. She's always lived as a Gladiator/Warrior and nothing more. Her country doesn't need her and she doesn't care about it either. Rebecca and Vivi aren't similar at all. I have no idea why people keep comparing them.
Barto I do love, and would love to see him as a Straw Hat, but as most people pointed out his role as fanboy would never work full time on the ship. If Luffy asked him to join he'd probably die of a heart attack
Yeah thats the one thing that needs to change about Bart. If he were to join he'd need a knew gag.