Just saying I'm kind sick of every single new female character automatically being considered a potential Straw Hat. We've had Marguerite, Caime, Perona, Boney, Shirahoshi, Hancock, Monet, Baby 5, Rebecca, that 3 eyed girl from one panel in FI and Viola all speculated as being future Straw Hat's from their first appearance when we knew literally nothing about them. It's kind of funny that we want a new female member this badly.
I share the enthusiasm for this topic though but I look at each candidate with a criteria. New Straw Hats need:
a dream, a position on the crew, a unique hair color (unless it's black), a unique first letter of their forename (we have L,Z,N,U,S,C,R,F and B already), a reason to join, a tragic flashback, a mentor, and they have to have allot of interaction with Luffy. Additionally they are also likely to be of another race like a Fishman or Giant as if you think about each member is very unique so there is thus allot of diversity in the crew. It's also likely that they have a unique fighting style or Devil Fruit power and are likely to use an "element" that we don't have yet (We have ice,fire,earth,lightning and plants so far).
Regarding personality there has to be a balance in the Crew as recently we have had Brook join who is more silly as a result some characters like Franky are a little more serious, which tells me that our next character is likely to be a more serious character that can act as a comedic foil to the silly members (On the more serious side we have Zoro,Nami and Robin) and (In the middle we have Sanji and Franky) and (On the more silly side we have Luffy, Ussop, Chopper and Brook) So one more serious character is seemingly needed, in the current arc Law fills in as the extra serious character but I don't think he'll join since, He's a doctor and his name starts with an L.
The final thing that'd have to be considered is strength of the new Straw Hat. We can clearly break the crew down into 3 groups (Luffy, Zoro and Sanji are the "warriors" of the group they are all really strong and eager to fight), (Brook, Robin and Franky are in the middle, they are all incredibly strong but less so than the other three and are less competitive) and (Chopper, Ussop and Nami who are still strong but not as strong as the others but that's OK they are not worthless since their talents are mainly outside the realm of fighting and also the main thing that holds them back is more there attitude or fear than anything else as when they fight they are formidable and are good tacticians).
In my opinion the next member is likely to be very strong but not as competitive as Luffy, Zoro or Sanji. So who do I think it'll be well Rebecca fills allot of the criteria but it's still not enough for me to be sure, I am sure however that Jinbei will and should join the crew eventually as he is serious, is water elemental, has had a ton of interaction with Luffy has a mentor and wants to join, not to mention his name starts with a J.
Oh yeah two more things I'd like to point out is that Luffy said around 10 members would be good in chapter 1 not exactly 10 and Luffy has changed his mind before, so the limit could be around 15 members (which is cool if you consider Vivi and Karue members like me). Also the age of the character is unimportant as Brook is 90 and Franky is 38. Ultimately all the members are in their own ways rejects and they find a place to truly belong in the crew. And that's all I have to say about it for now I hope this post helps your speculating let me know if you agree or disagree with me, it's always fun to speculate