I think Shirahoshi will be the next straw hat, if they get another from this arc. Shirahoshi has lost someone dear to her, Her Mother. She has had a tough past, though we havent gotten a real deep look into it. But still, if it isnt her, it will be NO! IT will be HER, im sure of it!
She is the daughter of Neptune, that might mean that he had someone teach her, or taught her himself, some defensive techniques, which to humans will be super cool. Having a giant mermaid might be the only way to combat San Juan Wolf, in one-on-one match ups againts the BlackBeard Pirates. She isnt as big as him.... but still. Maybe she knows MERMAID Karate? You never know.
Sure, she seems like a coward, but so was Usopp!
~Joudan Janai Wayo~