So, firstly the next character will be on Fishmen Island Please read it all, because if you won't read it ALL than that means you will throw all my work away, you will have no right to fight for what you believe just by throwing their ideas away like nothing, every bit from here is filed with some opinions and facts that happened, it has ARGUMENTS!! if you don't have any than don't comment!!! just writing "She's too big" or "it's gonna be Hodi somehow" means you're an idiot on this blog!
Oda really wanted to add the fishman species from the beginning, if I remember correctly he wanted to present a fishman from the 3rd chapter. After such a long journey and finally reaching the Red Line again, going to Fishmen Island and than getting to the New World, a new beginning, it would seem like Oda really wants to make the next member a fishman or mermaid, so that excludes all the theories about a new character from the New World. Luffy also likes weird and diverse people, taking in his crew a reindeer that ate the Hito-Hito no mi, a cyborg, a skeleton showing us how big the world is, that would mean approves of my theory, a fishman is for sure what Luffy wants as a new crewmemeber, he wouldn't leave the island without a 10th member, if he would than I wouldn't understand first of all why Oda even added the fishmen to the series?!?!? Trivia from [[1]]
Now I want to tell you why it won't be other fishmen than Shirahoshi I will begin with Hordy Jones: He is the main antagonist of this arc, being already severely cut by Zoro His power comes from those drugs that he takes. He used a nakama as a shield, despicable! His role-model is Arlong. You all know how Arlong is, also he would consider Arlong as his aniki. If your aniki would be beaten up and sent to prison because of someone, would you ever forgive those bastards, more than that JOIN THEIR CREW!?!?? I don't think so!
It won't be Vander Decken IX: He is also one of the main antagonists of the series. He without any remorse almost killed Hatchan, Luffy's friend. He also forcedly asked Shirahoshi to marry him for selfish reasons and because of him she was trapped in the Kouhaku Tower for 10 years, telling her it will either death or marrying him. Also Luffy is atm protecting her from him and already sent him in the ground. Also consider him joining the crew, his ancestor came to FI to gain control over the princess with powers to control Sea kings. Would you really think he'll drop that for his enemies, join their crew and leave without taking the Shirahosi with him in the worst-case scenario, it is even more close to impossible because Luffy does not forgive those who attack his friends, Franky was a special case, because he has a big heart and a tough attitude.
And also for those who believe Hyozou is the next one: He cares only about money, not about adventure and fun, so he won't be.
Hatchan has Keimi and his takoyaki shop, I one believe that after doing such horrible acts against Nami and asking for her forgiveness, becoming a pirate again, in their crew would be not feeling bad for what he did... also he is very wounded right now and won't be able to play a role in this arc. also he isn't strong, going in the New World would be really hard also to be added is that he can't play any real role in the crew, only getting wounded and fighting weak oponents(in the New World, weak opponents would even be stronger than him). The Sh crew is made out of greatness, Hatchan doesen't have it.
You all want the next member to be a girl, there are just 2 women in a crew of 9, it is the time for a new female member to join the adventure. I don't think you disagree with the next member being a girl then.
Now that I finished with the enemies it's time to say why it won't be Jinbe The arguments I will give won't be full-proof, even less for him not being the 10th member, because there aren't that many facts that make him "unworthy". Now: He get's shocked at allot of things that Luffy does, like bringing the princess out of the tower, he would scold him not to do such reckless things, now after the time-skip, would it seem fun to see Luffy get scolded for doing reckless things, except for swimming where Nami will beat him up? He gives you more of a captain aura. Everybody on the island calls him boss, he fought with Ace and they were tied, he would be just barely under Luffy at fighting skill and strength, don't underestimate a former Shichibukai. He would destroy the somewhat stable power in the crew, getting him into the crew would not be like Oda, he doesn't like to add people that don't really enjoy to have fun. I can't imagine Jinbe drinking, partying a whole week and laughing, or making jokes. I don't remember seeing him laugh at all threw the series.Even tough seeing him being scolded by Hancock was funny when he ate a bit from Luffy's food. But just as Luffy didn't want the two giants, as they were just so strong, or Ivankov, Boa Hancock or his brother Ace to join, because they (having in common) are too strong and they had something like leadership, they being actually leaders of a crew or army. Making a somewhat born-leader join the SH crew isn't in Oda's plans.
Now finally I will explain why it should be Shirahoshi: First of all... does she give you any charisma of a leader? She was kept in a tower for 10 years since the age of 6. She doesn't has any influence and isn't like Vivi, loving her country so much that she'd die for it(that's the reason why she didn't join the crew, ravaged country before nakama) so there wouldn't be a great reason for her to stay on the island, especially because she has 3 older brothers. In the arc, the story revolves around her and the kingdom(so still back at her). Luffy protects her, calls her cry-baby and acts on her as he did on Coby, and now they're friends. Her ties/bonds with the island aren't so strong(as shown till now) She is very good-looking!!! Sanji even controlled himself not to have a nose-bleed explosion, just to see her. The last time he saw her(we saw him seeing her in chapter 620 managed to control himself and act as he acts around Nami and Robin) this indirectly leading to Oda wanting her to be part of the crew, making Sanji used to her is a great start! Her size has no what-so-ever problem! If it was a giant that joined the crew? Than size would be a problem, but she is a mermaid, she can swim and now with her newly-presented power, that mysteriously fits perfect with resolving the problem of swimming in monster-infested waters. Controlling the sea kings, creatures of the sea that you don't wanna mess with, even Jinbe tries to avoid them, gives the SH a huge advantage in a sea-battle, eliminating the problem of fights against warships.She also isn't a sitting duck! Mermaids are the fastest creatures in the entire Sea, nothing can outrun them. Also did you notice that a weak character gets a stronger mind, learning how to stop crying makes you stronger than a person that doesn't cry from the beginning! I would like for you all to imagine how things might play for her to be next, but it seems like only Oda can do that! Oda worked a bit in making her character, inspiration coming from many tales(see her trivia) where the one she was inspired from is mostly a main character. You might not notice, but Oda preparing somehow to make her the new SH. Losing your mother, being threatened and now Luffy saves her from all of this, just like he did with Robin and Nami! She might be weak at strength, but with such power, controlling the strongest marine creatures is absolutely incredible. Anyone from the marines with a rank smaller than vice-admiral would not be able to beat a Sea King(except for Smoker, as he is worthy of having a higher rank than commodore) also mostly captains, with strength like those of the New World can face Sea kings and win, but there would definitely be a hard fight to win. It's not goo to play with these monsters, and fighting more than one makes it even HARDER!!! Sea King have huge sizes, being bigger than the kracken, it might be possible for some to grow to sizes of even islands!(Oda showed us a shell and turtle that were mistaken as islands so there is no reason not to believe that there might be sea king as big as one too) so that means that she isn't useless AT ALL! And making her a SH, it isn't a bad idea, having a crybaby in the crew is something new and interesting, Luffy making her cry and Nami beating him up for that would really be funny:)), but that's as much as I will tell you about my imagination, I believe you don't want to hear ALL that I imagine Oda might be up to(and that would destroy the whole Arc), this would seem(at least for me) that Oda also wants her to be the 10th and it would seem like Jinbe was the first thought, but things turned out differently as what he'd planned during the war so, after the time-skip he decided to make her join(The last part had no facts or solid arguments so you could disagree easily, it was just opinion, if you don't like it, ignore it!) Also I am sorry for not giving you more arguments, After 1 h of writing and thinking, I can't put the ideas into order, forget them and might even start saying stupid things. Now I want real arguments!