Final Villain will be either BB because he is a 'D' who is continuously challenging/provoking Luffy(not to mention he was the primary reason why Ace died), give Shanks his scar and basically got 2df which nobody has in OP, wait he also destroyed Rev base and killed his previous captain and defeated his crew(enough reason to be the main villain of a series) or it will be Gorosei with Admirals backing them(since WB said "one who finds OP yada yada yada declares war on everybody ").
Kaido's hype is nothing new. We already have WSM, WSS, Gods etc. He was already defeated 7times by Marines and other Yonkos. So he doesn't give that kind of vibes which makes him the final villain of the series.