As the rule of shounen go, I see Imu as the final boss, and the final boss of shounen is almost always a fighter.
I don't see BlackBeard being the final villain, in the current flow he will be a finalist in the pirate king run, maybe reach Laugh Tale at most, and end there, while Luffy goes to Final war;
for BB to be the final villain, he needs to reach Laugh Tale + Defeat the WG, and Luffy will be doing what exactly while these two antagonists throw hands?