I think at the end of the series the marines are all at raftel icluding wb pirates, bb pirates, and wano swordsmen, and 4 emporors and thats when luffy meets Shanks again and fights bb pirates. Both crews will get more crewmates. I think It will turn out like this.
Luffy vs Blackbeard
Zoro vs Shiliew
Nami vs Van Augur
Usopp vs Avalo Pizaro
Sanji vs Catarina Devon (finally agrees to fight an ugly girl at least)
Chopper vs Doc Q/Stronger
Robin vs Vasco Shot
Franky vs Lafitte
Brook vs Jesus Burgess
???? vs Sanjuan Wolf
The reason I chose these people to fight each other was because their fighting styles dont necessarily match from what it seems. And once WB pirates and Straw Hat Pirates go against the marines, i have few ideas about the one piece thing:
WB pirates say go and find the one piece so they risk their own lives for them. And Luffy agrees. And soon he finds the one piece
Luffy dies and Zoro takes over, burying the one piece with him. And before he dies he tells Zoro to give his hat to Luffy but Shanks tells him to keep it. And then his lasts words are telling his crew to complete their dreams.
Luffy fins the op and the crew splits up.
Luffy decides to not open the chest or whatever and decides to move on with his crew so his dream wouldnt be over.
AND OF COURSE SABO APPEARS!!!!