BB definitely won't be final. Imu is final. Big Mom coming back with a Kaido homie is on the board, but I think its extremely unlikely at this point. I feel like Oda is done with BM. But for Elbaf to be a big arc, a major villain lineup is mandatory. The final war is definitely gonna be the final, after Luffy becomes the pirate king, so that rules out the Gorosei and Imu, I mean, them mere fact that Oda didn't allow a single Gorosei to go down in Egghead, not even Saturn pretty much confirms that Imu and Gorosei are final. Shanks has connections to the WG, and with his parallels to Roger and connections to the WG/cloning stuff going, he'll also most likely be either Laugh Tale/Lode Star or final war. So that leaves really only 2 major forces that can take the spot. BB and Akainu. Of those 2, BB makes more sense since the BBP were much more active in Egghead Arc, but at the end, we could get a "fine, I'll do it myself" from Akainu. Unless Luffy is fighting Mihawk there's no chance of Cross Guild either, Oda will never allow Luffy to have a meme fight whilst the other straw members get actual fights. So it really only is BB or Akainu, of this BB sounds the most likely, off course the mysterious man could throw a wrench in it, but unless his Shiki or somebody strong from Roger's era, don't bet on it, even then I don't see him being a villain. Which leaves BB. There's nothing stopping the straw hats from going to fight BB, in fact this is the most likely time, at this point in the story he can still be a threat. Won't be true for much longer though since OG Joy Boy, Shanks and Imu all are on a completely different level in regards to haki and BB can't match up to that with his hax, the only time he can be a meaningful threat is now, whilst Luffy is fruit spamming, the moment he gets haki buffs BB will become too weak to pose a threat. So in this regard, BB will definitely be the soonest to go down, maybe Akainu, but with the trajectory of the story going towards haki buffs, there isn't any space for BB to pose as a fearsome villain anymore, unless Oda plans on making BB and Aokiji 2v1 Luffy. Other then that, I think BB will serve as a catalyst for Luffy levelling his haki up.