It looks like people didn't understand what WB's function was in the series so you are searching for a replacement.
''Sat before the throne but never upon it''
WB was a placeholder for Roger's throne until he gave up on his seat and told his men to follow Luffy instead during the war.
Does Luffy need a placeholder after he dies? Thats what you basically asking. Pirate era will probably finish after Luffy dies so no need for a WB replacement.
Roger's greatest foe was Xebec not WB according to Sengoku:
Then second was WB for a while until Roger become PK, then it was over.
BB is the combination of both Xebec's and WB's legacy. Only that BB won't be a placeholder like WB after Luffy dies. And there won't be any need for placeholder as well since everything will be different.