Swapping out Straw Hat Pirates for new crew members may improve the lineup, but loyalty to Luffy's dream is irreplaceable. Potential replacements for current crew members include Yamato, Bepo, Baby 5, ...

Dreams are one of the biggest themes in One Piece. One way in which this is expressed is the way that every member of the Straw Hat Pirates has a big dream that they're working to make come true.