There are rumors that the next crew member will be a former villain.
Bon Clay: it would be awesome if he joined, but he is almost certainly dead.
Arlong: whoever said this is an idjit.
Jinbei: He promised to meet up with luffy, and is indebted to him. Plus, they are going to fishman island.
Buggy: I personally would love to see this. he is no longer luffy's real enemy, and they have fought together. It would be very interesting to se him on the team. however, he has his own crew, and i cant see him leaving his new allies from impel down.
mr. 3: Also an enemy, but he is part of buggy's crew now, though on better terms with luffy.
Boa: she cant realy leave her position of shikibauchi.
My money is on hatchan. he is a frined now, a former enemy, and on fishman island, giving luffy and his friends a chance to meet him again.
the only other characters i can see joinging would be coby and django, but theyb are in the marines, which means they cant.