Another concept is the exploration of different worlds within One Piece. If an admiral crosses over to a less - known or mysterious island, it can open up new storylines. Maybe they discover ancient secrets or new types of Devil Fruits. This can bring a fresh perspective to the One Piece universe as a whole.
Well, the character development is a big part. In these crossovers, admirals might find themselves in situations that force them to change their views. Like if they are paired with a more morally gray character from another part of the One Piece world. They could start to question their strict sense of justice. This adds a whole new layer to their personalities.