I wouldn't include Brook here, the guy has had some great moments throughout the New World and he was the MVP in Whole Cake Island. I don't think we can say that the character who first faced an emperor among the Straw Hats, stole her poneglyph, broke Carmel's portrait, subdued an army of homies... plus other small yet interesting things (the concert in Sabaody, using his new abilities to free them from the cage in Fishman Island, finding Kin'emon's torso and defeating one of the dragons, defeating Giolla...) hasn't been a main player in the New World.
I think the problem with this quartet are Robin and Chopper, mainly. Robin has been quite a plain character who doesn't really have anything to do apart from reading poneglyphs, so the spotlight will come by end of series; and we all know that Oda basically sold Chopper to merchandising and flanderization, and on top of that he has consistently introduced characters who are either better doctors or have some kind of curative ability that renders the reindeer virtually useless. They both better do something, because they are harming the group as a whole, in which as I said Brook has been a main player so far and Franky is in the middle, more or less.
Caribou asked the same thing and the poor guy isn't assumed as a member by default. There's a big difference between considering Yamato as a potential crewmate and literally including her in tons of threads that revolve around the crew in its current condition.