I definitely agree hes a pretty awesome antagonist, but I wonder if he really is the final one. A lot about Blackbeard screams "epic" milestone rather than an emotional finish. Like I see a fight with Blackbeard being the most epic, especially due to the sheer variety in his crews broken powers so far (and we barely know any of them), but I dont really get an emotional investment from him when it comes to Luffy fighting him. Really the same with Im, a character we dont even really know yet. I dont think Akainu is remotely the final antagonist but Luffy taking him out would be way more emotional since he murdered Ace.
This is why I think it all comes back to Shanks: the hat, the emotional ties between Luffy and him, Mihawk and Zoro, Usopp and Yasopp. To me this fits way better as an emotional climactic battle, but only when we know what Shanks is really doing or what his goals are.