I think their purpose is to be the final pirate obstacle for the SH’s before the war against the World Govt. I think they’re as strong as any other yonko crew, I believe in Blackbeard acquiring a third fruit to make himself the strongest yonko by the time Luffy faces him. I believe they will defeat the Red Hair pirates in battle. As far as the tenth member I’m suspicious about Urouge and Carribou’s role in the story, outside of them I have no idea who could fit. For their matchups I’m thinking: Luffy-BB, Zoro-Shiryu, Nami-Lafitte/Pizarro, Sanji-Pizarro/Lafitte, Usopp-Augur, Chopper-Doc, Robin-Tenth, Jinbe-Burgess, Brook-Vasco, Yamato-Devon, Franky-Wolf. Not really good at guessing bounties.