The series will end with Luffy, by himself, climbing up to Twin Capes Lighthouse. Crocus will recognize him and say nothing, then whistle for Laboon. Laboon will come, and Luffy, smiling, will stare at Laboon.
Luffy: "I said I'd be back, didn't I? Ready to finish this?"
As they start fighting, it will end. Or even as Luffy throws the first punch. Because the outcome of the battle doesn't matter, just the fact that Luffy kept his promise does. In the last panel, Roger's final words will be displayed.
And the saddest moment for me in One Piece is when Robin was in the boat, and forces herself to laugh. Her Dereshishi is heartbreaking…
It reminds me of the end of MGS3. Being forced to kill The Boss was so difficult.