Hmm. The theory that it is Sengoku on the ship with Tsuru also makes sense, the biggest clue being the line about okaki.
I'm still hoping it's Garp, only because I want to see his interactions with Luffy, Sabo, and Don Chinjao.
If it's Sengoku, that could be interesting as well. After what...
I think it's heavily implied that Tsuru is going to finish capturing Doffy.
Garp[?] says to Tsuru: "You've been chasing this crew (Donquixote Family) for quite some time"
Tsuru tells Garp[?] in annoyance: "On your orders!"
During Law's flashback Tsuru was the one chasing the Donquixote Family...
