I think Luffy and Blackbeard will defeat Sakazuki and Shanks respectively before facing each other. I don't know where I'd fit Imu, but it's a toss-up between Blackbeard killing "him" or being one of Luffy's last major opponents. Either way, Sakazuki, Blackbeard, and Imu go down before or during when Luffy finds the One Piece. I don't see the Admirals or Gorosei being proper matchups for Zoro and Sanji.