Also are you sneakily confirming that Carrot is joining by not including her amongst the offboard crew? Because it doesn't get more "want to explore with them and have fun" than stowing away on the Sunny to go on an adventure with the crew.
Nah but part of me still wonders if Oda is gonna have her be hidden somewhere or catch up to join. This is not because I have a desire for her to join. I just think there was a logical argument for her to join based on Oda's own fancies and the proximity to the end.
Fair enough. Not gonna dig deeper into that last statement because I would be going too far into the my personal understand of Oda's thought process route rather than making a fair assessment judging hard objective facts.
I'm still not buying the entire onboard/offboard crew thing. I'm not saying that you are not telling the truth. What I'm rather having a tough time with is the value of such a concept. Especially for the Japanese audience.
I'm really gonna try to keep it brief with this without mentioning specific experiences. But ever since I coming to Japan I started seeing One Piece in a different light. To make it short, One Piece here is pretty much something like Doraemon. It has it's core audience BUT it also is so omnipresent as a long running series that many people know of or have heard about it. The latter group is important here.
There's so many ads and shows aimed at a more general audience than the hardcore One Piece fans. And those always focus on the Strawhat crew which are the guys you refer to as onboard crew. The point I'm trying to make here is, that from personal experience I can tell that people who don't follow One Piece can name most of the crew. They do struggle at Jinbei though because his appearance in such group shots is new. In other words, only people who follow One Piece can actually make sense of Jinbei at first sight.
So having an offboard crew would surely have a lot less mass appeal because those characters don't really ring a bell for people who are not following the manga/anime. If you'd for example see Yamato with the crew, somebody would say "oh, I guess this character is part of the crew now". But having her or Vivi be offboard crew members just means nothing.
I'm fully aware that of course we shouldn't cast the actual followers aside when speaking about the series. But as you yourself are even more familiar than myself, One Piece is just so omnipresent here, that there is a crapton of things created just for the sake of appealing to a broader audience. And the Strawhat crew is at the center of this. So I really don't see what the appeal of an offboard crew is to the production team.