First of all luffy said in the beginning that there were only going to be ten strawhats and even if they were to get another member it would be Bonney 100 percent. And also scooper gaban has the last poneglyph kuzan said that the man marked by flames has the last poneglyph and scopper gaban is that man, the strawhats already have the other three, the one from zou the one from big mom and the one from kaido. They just need scoppers.
The shandians were just ordinary humans that got pushed up into the sky via the knockup stream then they probably co evolved with the skypieans and that’s why they have wings
