As big and multifaceted as the Wano arc is, I have a hard time accepting the premise that "only one 'new' Strawhat can get the requisite development to join this arc"
It's not that only one can, but that the story does not seem to be building towards anyone but Jinbe joining. Oda could make several characters shine through Wano, building them towards joining the crew, but he hasn't.
You cited Water 7, but Usopp, Robin, Franky and Merry all had their character arcs there. Plus, we entered the arc knowing beforehand that the crew needed a shipwright, so there was the expectation that one of the several candidates (Kaku, Paulie, Franky and whoever else was there) would be joining the crew.
Wano, on the other hand, didn't give us any such thing. Carrot, which was the best candidate, was just forgotten in the background. Tama never got developed into a possible crewmate with crazy abilities. The character that has been built towards joining, and finally has, is Jinbe, and only him…
And then there's Yamato, which is a very weird case. He appeared for the first time when the arc was past its midway point. I still can't believe that Yamato wasn't foreshadowed before. Tama could have mentioned another friend of Ace, we could have catch mentions or even glimpses of him in the Onigashima. We could have been led to believe he was a "fourth calamity" (long speculated by fans), only to then find out he was a girl and wanted to help Luffy.
So far, Yamato's purpose seems to be the caretaker of Oden's Journal. That's why the author created him, it seems. But, of course, Oda can still pull out a flashback, give him a fight and develop him into something else. With what we've seen so far, however, I'd say Yamato will stay behind when the arc is over.
--- Update From New Post Merge ---
@Vongola_Boss_XI:
I think it's all about the story and the characters.
It is about story and characters. But an author tries to make main characters as unique as possible, so that each is as remarkable as they can be. You can see that in Oda himself defining things like character's colors or position in the family. Things like color theory are indeed secondary, but they help trying to single out certain characters that are more likely to be real candidates.
Again, I only brought up the violet/purple thing because this week's color spread not only has a lot of it, but also seems to confirm that the theory was right about Jinbe's color.