The Stag Gifter. Nuff said.
Just kidding. I will give my serious choices.
Wyper - With Usopp doing away with dials, I feel we need someone who is capable of using them in battle. He is the right guy for the job and shown to be quite grateful for what the Strawhats did for Shandia. Unfortunately, he will forever stay as a guard in his land instead of ever leaving the very thing he fought for. Not to mention his people he loves dearly.
Marguerite - She has so much chemistry with Luffy, being the first member of the Kuja Tribe that Luffy ever meets. Hancock is the preferred choice, but I think she would stay being a leader of her tribe than being completely subservient as Luffy's underling. She has potential to be an underdog fighter that grows stronger with advanced forms of Haki. Maybe get along with Nami and Robin too.
Gaimon - He always seemed like Strawhat material since day one. Even if he wants to stay with all the unique animals on his island, he would feel right at home being the treasurer of the crew. He does not need to the strong or even fast, but he works well for the role. You could say he is the perfect spy for espionage missions with his bush-like hair.
Bon Clay - I know what some are thinking. "Bu-but we already have a kicker on the crew and he is just Sanji in drag". Okama Kenpo is uniquer enough of a fighting stay to make him standout. His Clone Clone Fruit when used more offensively could be a great asset for the crew. Not to mention he is their friend, did major sacrifices for Luffy twice in a lifetime. Perfect but unlikely crew candidate.
Princess Vivi - A controversial and debatable one indeed. Yes we know she is an honorary crew member, but she had been separated for two years to look after her own kingdom. Everyone minus Zoro were still heartbroken that she could not travel with them any longer once Alabasta is saved. With recent events regarding Imu and an assassination attempt, we are unsure if she is fully kidnapped or traveled with the Revolutionaries. She could possibly rejoin the crew once more, but this is Oda. A guy known for his curveballs against the expected. I wouldn't hold my breath on calling her a lock.