A good writer is going to leave little open ended things like that sprinkled throughout his or her story. He doesn't have everything planned but he leaves himself openings to fill in when needed. If everything was planned out ahead of time we'd see things like Brook during Laboon's flash back at the port or Bartolomeo at Logue Town. First time the Warlords are mentioned the silhouettes are all off from their final forms to. Think they all had swords. Now that could be just someone's thought of what a Warlord could be but still.
I'll give you Bartolomeo and the Warlords.
But Brook seemed planned out. Laboon felt too random at the time, and we eventually got one come Thriller Bark.
Or one could assume it was a bit of world building, saying "this world has giant whales".