This is just a prediction of mine, since we know Luffy will chase Ace all the way to Marine HQ.
I have a Strong feeling Luffy's nakama will follow him there after learning of Ace's Execution and they will do something so epic that no Pirate has ever done before (just guessing)... sail to the New World through Marine HQ.
That would be so Epic!!!
It's about time Luffy and co does something crazy epic again... as if infiltrating Impel Down wasn't enough, he goes and sails to the New World through Marine HQ and skip Mermaid Island all together.