Maybe Saint Shanks vs Luffy happens first and BB fights the RHP after they are exhausted from fighting.It would make sense too,Elbaf seems to be Saint Shanks territory.Both Luffy and BB should go there for the final poneglyph.
Saint Shanks left elbaf.
He came to wano for BB.
He mentioned BB against his fight with kidd.
He will go out to find BB
BB will head to elabaf...gets rpg and head to laugh tale.
Saint Shanks will stop BB - they fight.
Meanwhile Luffy will reach elabf - gets rpg and learns about Saint Shanks defeat.
He moves towards Laugh tale - fights with BB