I remember in the weeks between "Kaido's son" first being mentioned and Yamao's reveal, some people speculated that Yamao would be some ugly fat meme as a troll since we'd expect Kaido's son to be a total badass. It seems like Oda genuinely considered the idea based on this concept design for Yamao:
Oda would be fully aware if a character was going to join the crew during the designing process. Does it seem likely that Oda created that design with the belief that it could possibly be the appearance of a new nakama?
To clarify, OP is obviously an extremely quirky series full of all sorts of crazy designs, but this isn't just some unusual design. It's straight up the type of character design you'd give as an intentional meme. A gag subverting the expectation of Kaido's badass powerful son with some ugly fat dwarf.
You could say "Well Oda didn't go with that design in the end so it doesn't mean anything at all." But the reason that isn't the case is that Oda
considered the possibility of Yamao just being a joke character. If we believe that Oda prepares well in advance and knows who he wants to be the next straw hat, then it would be unusual for him to make such an immense change.
We know that Vivi was originally meant to be a one time side character, but Oda saw an opportunity to make her the princess and so he played into it, knowing full well all along she was never actually going to leave Alabasta with them. Same deal with Law. He was supposed to be a side character, but Oda decided to start building him up into something more, once again knowing that it was fine since ultimately he'd never actually join the crew.
The fact that the ugly troll design is completely unrecognisable from the Yamao we know today suggests that Oda didn't have a firm grasp on the main identity of her character, which most likely wouldn't be the case if she was always destined to join the crew.
Unsurprisingly though it seems like Oda always know the purpose he'd like Yamao to have. She was originally going to be named "Ninigi" after the deity who was sent by the sun goddess as an emissary to Japan to bring civilisation, justice and order to humans.
https://mythopedia.com/japanese-mythology/gods/ninigi/
-Sent by the Sun God(dess)
-As an emissary to
Wano Japan
-Had a pivotal role in building up the country
-Results in immigration to Japan
I'm sure you can draw the parallels. These ideas are all still present in the current Yamao with her guardian of Wano devil fruit, her attack names referencing the god of nation building and other guardian themes, etc.