It could be that Luffy will get to the plaza and reveal to the people that Kyros, the great hero of the colliseum, is alive and that he is Rebecca´s father.
One more possibility is, that Luffy is heading to Sengoku and thank him that he gave Ace´s Body to the pirates, so Ace could have a normal funeral. That would also fit to Luffy´s Charakter.
There's no way he's going to do anything related to Sengoku. he's going to to something related to Rebecca or Kyros. As of what, we'll find out!