No one this arc...I hope. As much as i like Monet, she'd be Robin nr 2 in the crew. And there is no one else I could see joining. Monosuke and his father won't join a pirate crew, unless Monosuke's role in the story is bigger than I thought (he does know Don Flamingo, but idk if that's enough). Maybe they will travel with the straw-hats until Flamingo is taken care of? If he really is the next major villain. One I could see traveling with them is Law, though just as an ally.
Jinbei already has a spot reserved for him.
Dang it Avalon, you c(XD LOL)-block Shirou and Reinforce, but don't protect his mind in other ways? What is wrong, you woman?
Friendship, be made! Magical power, gather! Starlight Breaker.... this world!