We know via Greg that Oda's original concept for the crew was fulfilled with Jinbe joining, true.
But "original concept" can be many things. We know the crew was quite different in earlier sketches, and was moving towards what we know today as the series began.
A few years back, a book (I think one of the Color Walks, but don't quote me on this) brought two early sketches of the crew:
Here's a larger version of the second one:
The first sketch predates One Piece by a few years (3 to 5, can't remember). There's no Robin, there's a botanist, and the Shipwright is that midget figure.
The second sketch is from about the time One Piece began. You can see the crew almost as it is today. Robin is there. But the skeleton musician is called "Skirt" and was meant to join before the shipwright, who has its name scratched.
What's interesting is that in both images there's erased characters. You can clearly see some remaining lines from the original drawings. That was done because Jinbe had not joined yet.
So, after Jinbe joined, OP Magazine released the full version of the first sketch, now with the fishman shipwright, named "Saigo", who was the proto-Jinbe (Jinbe was originally meant to be a villain, but he was merged with Saigo to become the character we know today):
What's interesting is that, despite the crew seemly "completed", they never released the full version of the second concept image, the one from back when serialization began.
There's a lot of empty space on the left side, as well as the height lines are erased there, indicating the space wasn't so empty originally. There could be at least two, if not three, erased characters there:
Plus, if you count the characters, the image is unbalanced: you have 3 (4 with Jinbe) in the left, and 6 on the right. Isn't that strange? Why not go for 5/5, if there's only one character left out? Maybe it's because there's either 5/6 or 6/6 characters there, making the image even.
So, it may be that the 11th Straw Hat was planned for a long time.
(Not to mention Luffy was already wanting "10 people" in Chapter 1...)