Akainu holds more emotional impact to Luffy's character, being the one who killed Ace. Whitebeard's words also made it clear the final arc won't happen until after the One Piece is found, which makes Blackbeard unlikely. The only problem is making Akainu strong enough to be the final villain, since unless he's gotten far stronger these past 2 years, he won't prove as much of a challenge as a Yonko with multiple DF abilities. My guess is Vegapunk will give him some cybernetic enhancement that results in similar abilities to what BB has.