Wano, tbh. It’s where Luffy became an Emperor, the rest of the crew all had multiple adventures leading up to it (even the late comer, Jinbei, had been with Luffy for four arcs prior to Wano.)
Luffy‘s Dream, as a chapter, was a pretty definitive moment for me. Anyone coming after that has missed too much.
I reread the series from Zou to the current chapter in the past few months and while I like Bonney, I don’t really think she will end up joining. She’s not a bad suggestion, but if I have to come down on a side it’s no.
The only one who’ll fit in again is Vivi as 5.5, due to the buildup Oda’s done for her and Imu.