@Monkey:
1. Asserting for absolutely no goddamn reason that the crew has to be complete before Wano.
2. Asserting that Wano is next (forgetting completely about the group going off to find Marco).
No goddamn reason? Obviously, I was thinking Wano was next and the whole crew should be there before they take down one, that's my reason. But certainly, as you are saying, the Marco retrieval story line might just get a plot by itself (to me, from Neko & Inu's explanation, it seemed this plot would occur in a flashback before Wano, but you may be right), and as such, that arc could give the Straw hats one more opportunity to get a new crew member, contrary to my belief that Cake Island is definitively the last stop before Wano.
–- Update From New Post Merge ---
-Enter the Grand Line
-Defeat their first Shichibukai
-Experience their mega existential crisis of breaking up
-Experiencing the Going Merry before it died
-Experienced the absolute defeat on Saobody
-Crossed the Red Line again
Surely THIS arbitrary landmark in the series will matter this time though!
Personally, I think defeating a Yonkou is a much less arbitrary and more important landmark on the Straw hat's journey than you claim. Each Yonkou seems to have a military capacity capable of taking on the marines on their homeground. You are telling me that the removal of such a big player wouldn't be pivotal to the plot: it would establish Luffy as one of the big players and leave a huge power vacuum in the New World. How is that not significant compared to saving a single country (Crocodile), stopping a zombie army (Moriah), or massively hindering arms trade and once again, saving a single country (Doflamingo)? Case in point, Luffy's defeat of Kaidou will produce effects on far greater level than ever before: Luffy will be declared a serious contender for One Piece and the world will be in turmoil? How is this an arbitrary landmark One Piece's story line?
Now, back to the primary discussion. I might be wrong when I say this, but I think it would be strange if Straw hat crew isn't fully formed before they stand on top of the world. Wouldn't it be bizarre if they just tacked on a crew mate after they proven to be a top tier pirate crew? But I digress.