Let's see
-one of the major reason for ace's death
-breaks the rules of the verse by eating 2 devil fruits and that too 2 op ones at that and might potentially have a 3rd
-is a parallel of rocks D xebec who was said to be the biggest rival to roger.
-has the ambition of becoming the king of the world and a potential final villain
-only villain that understands luffy and agrees with his ways
-Rose to the status of yonko before luffy and has a former admiral under him
-is about to get his hands on 3 poneglyphs, and a person to read them just like luffy
-luffy's df is said to be only restricted by imagination, bb is the only person that can actually nullify that and force luffy to win on haki alone
As I said in my prev points, bb has a big chance of becoming the final villain. Its a 50/50 between Imu and him as both thematically