I was also wondering about the "10 people" Luffy quote myself not too long ago, so I looked at the Japanese and Luffy says, without equivocation, "I want ten people!!" (「10人はほしいなァ!!」) He doesn't say "almost" or "about" or anything like that, which surprised me. But Luffy's a man who knows what he wants! [Corrected by Aohige below.]
@Dixxy:
1. Does the ship count as a crew member/Straw Hat? Specifically I'm referring to the whole "one of the Straw Hats will die" thing from a Jump Festa several years ago that ultimately turned out to be the Going Merry's demise.
2. Is Luffy including himself in the head count?
If the answer to both of those questions is yes, there are ten Straw Hats. Is they are both no, we have two more left. And if one's a yes and one's a no, we have on more to meet.
1. I believe that the ship counts as part of the crew, but that it shouldn't be counted as part of the 10. Luffy said he wanted 10 "people", so he probably wasn't accounting for a ship when he said that.
2. People seem to be fairly divided on this. I personally think that when Luffy said "10 people", he meant a "10-person crew" that would include himself. 10 would seem to be an easier number to work with (from Oda's standpoint) than 11. And if they end up with 10 people, they would end up mirroring the Blackbeard Pirates as well (assuming that Doc Q and Stronger come as a set). If this is the case, then we only have one more Straw Hat left to add.
@Dixxy:
If asked, I don't think Oda would respond "There are ____ Straw Hats we have yet to meet", but once the crew is complete do you think we would get than answer either in story (i.e. Luffy saying something along the lines of "wow I finally got my ten people") or an outside interview/SBS ("Yes, Newbie McNewguy is the last Straw Hat the crew is now complete")?
My friends and I have discussed this before and some of them believe that the crew may be complete.
I feel like since Luffy apparently knows exactly how many people he wants, when that number is reached, he'll say so. And looking at it from Oda's POV, it's best if he lets the audience know when the crew is complete. Otherwise, readers could be left waiting for new additions that will never show up. Plus, it would make for a great "this is IT" moment – sort of like the Loguetown barrel scene you mentioned. (This is all assuming that Oda knows how many crewmates there will be, no question. I think he does.)
Based on all the above, I really feel like we've got one more crewmate left to go, and that they'll join before the New World. "5 from East Blue; 5 from the Grand Line" lends the whole thing a certain equilibrium. Luffy can announce that the crew is complete, which gets us even more psyched for the New World, and then Oda can work on developing crew interactions from there on out.
I think the second most likely possibility is that the crew is already complete and that Luffy will say something to that effect when they get back together. (9 people + 1 ship = 10) They've been through so much now that I wouldn't be surprised if this happened, either. And as you said, there hasn't been a need for anyone else since Brook joined. If a new person joins, they'll need a "role" -- Oda could easily create this but hasn't yet, so his intentions aren't clear. (And the potential for an "awkward" tenth joining is there, but really, that's all up to Oda in the writing department. If Oda does a poor job, it'll be awkward; if he does a great job, it'll be awesome.)