Their purpose is long overdue. The match-ups are over. Big Mom's already defeated. We're at Wano's boss battle. And when Onigashima lands, Oda must deject of them somehow.
Option A: They remind the Straw Hats who they're still dealing with but flee like Gato's henchmen from Naruto at the end of the Land Of Waves Arc for reasons I don't feel like listing.
Option B: Kizaru and the Marines show up and clean house with them.
Option C: Not sure how Oda would make sense of this, but after their recovery, Big Mom promises to leave with her crew if Luffy beats her one-on-one.
Option D: Something about O-Lin.
Option E: A rift among the Big Mom Pirates.
Option F: Anything else (can include any combination of the other options).
Oda could've at least had Smoothie, Compote, and Daifuku tag along with Perospero to Onigashima. Shachi and Penguin team up against Compote, Heat and Wire team up against Daifuku, Bepo goes Sulong to beat Perospero, and Yamato handles Smoothie. Meanwhile, Snack and the other Charlottes stand guard for Queen Mama Chanter.
they aren't going to be relevent, they had their arc, their captains been defeated, the story will move on. Oda will not waste time on the old era anymore. Oda will take whatever option that keeps them out of the story
I think they will save Big mom and run away when the marines show up and start arresting the defeated. Big mom for me is one if those grey villains who won't ever truly be defeated or arrested. Oda is too sensitive for that given she has kids around tenish years old. The only thing going down is her reputation and possibly status as a yonko but she will still be a great pirate non the less.
it depend if wano open the border the news about big mom defeated will shock the world unless WG try to cover that up
but BMP likely split faction katakuri vs perospero faction will begun civil war
so far it,s depend on oda want but it feels BMP is not use Fully by oda