I think either Jimbei, Sabo or an unitroduced character should be the 10th member of the Straw Hats.

This is already a moot point. He's been invited, and he accepted.

Huh. I guess that's one way to look at it. I don't consider Jimbei a member until he officially agrees and sets foot on their ship. He delayed his answer, so as far as I'm concerned, he's not a crew member. Yet. It's a different situation, but it's the same line of thinking I use when I consider Nami to be a part of the crew after Sanji and before Chopper, rather than third after Zoro. I once jumped to conclusions--then Vivi happened.

So I'm going with a yet to be introduced character--hopefully a female warrior type strong enough to break the combat ready trio of the Strawhats. I'm guessing Jimbei will return in time to be number 11 or 12., assuming something doesn't come up.

While they all seem plausible candidates, with the exception of Sabo, I really don't think the Strawhats will get a new member after this arc. My reason being, well for me anyways, nothing has really happened that pointed to any of these guys joining the crew. If we recap, the only Strawhat so far that didn't have a really logical reason for joining the crew is Robin. All the rest were either saved or something they cherished was saved. While Bartholomeo would join on a whim if Luffy asked him, i really don't think Luffy will ask for him to join the crew. In the case of Rebecca, she does have all the qualifications to join the crew. By this i mean, she has an interesting backstory that revolves around a lot of traumatic events, is unique and skilled in her own way, and has the potential to get stronger as the story progresses. But i doubt that she will be member #10. Lastly are Jimbei and Sabo. First of all, Sabo is the second in command of the Revolutionary army and second of all he already implied that he wouldn't join the crew. He did say he would help Luffy if he ever needed it; so in a sense he was saying that he would help Luffy as a third party but not as a member of the crew. Plus he, Ace and Luffy all made a promise to become captains and rivals; so I don't think he will join the crew either. Then there's good old jimbei. Though he already said he would join the crew in the future I think its still too early for him to join. I mean only a short time has passed since he said it and I'm not sure how well it would work out plot-wise if he joined the crew only 2 arcs after he kinda turned the offer down.

I suppose it depends on whether you think Oda will eclipse the 10-person total. I could see there being quite a few interesting characters they could add, and I agree, BP, that a female warrior-type would be a good addition (especially since the crew is rather male heavy at this point). If we look at it your way, that Jimbei hasn't joined the crew yet, then you're right that it's possible. It's just not something I'm clear will happen, since 10 seems to be a hard limit.

Luffy and Sabo should be in yhe same crew.