One thing about the fights after this arc, is that regardless of who it is Shanks, BB, or Akainu, the fights will have a lot more emotional resonance because Luffy has a history with them and a strong personal reason to fight them. Because of that the fights will be riveting, I think more so than anything we've seen yet, so I don't mind who the "FINAL" one is TBH.
However, this is just my gut, but for some reason I think BB. I don't know how but I just get the feeling Oda is building him up to be it. I actually see the final battle not having one faction verse one faction but multiple factions fighting for the future of the world. Like BB v WG v SH. So maybe BB gets to the one piece first and Luffy is doing his best to catch up and needs to beat him in some grand final war. IDK just spit balling, i'm just along for the ride.