Many reasons. Made a video on why Akainu is next:
Many months old, so not completly up to date anymore. Blackbeard can't be the next villain because he hasn't even dealt with Shanks yet. He doesn't seem strong enough for that. Perhaps a third DF is still missing.
Barto, Law and Jozu are the Ace 2.0 trinity:
Executives/Executions
Spade: Pica/Ace, Heart: Corazon/Law, Club: Trebol/Barto, Diamond: Diamante/Jozu
Not sure if Law really escaped. If Blackbeard still gets him, it would make sense for him to hand Law over to become King of Pirate Island. Shanks will probably beat Barto up and thus enable the navy to easily arrest the cannibal.