if jinbei is in his 20s then he will join. i dont think jinbei age has been revealed yet. boa is the youngest is ans she is 27. moria, mihawk, crocodile. and doflamingo are at least as old as shanks and he is 37. BB is in his 40s at least and kuma we dont know his age either.
they wanted ace to be a shichibukai and he is pretty young so is dont see an age requirement.
i say all that to say i dont think we will get another stawhat that is not 2 far away in age from luffy. this is the breakdown of age
chopper-15
luffy and usopp-17
nami-18
zoro and sanji 19
robin 28
franky 34
brook 88(dont know if his life started over or not but this is his total age)
sincw the strawhats are such ayoung crew i dont think they will get an apprentice, if anything chopper is the apprentice from him being so naive to the world. i hate to say it but brook is the old head in the crew and i guess sort of the wise one since he has been a pirate longer than the others, so i cant see another person that old joining the crew such as a rayleigh.
i also cant see another female joining the crew. we all know that the strawhats and the Blackbeards will duke it out in the end. i mean theie crews are almost mirror images of each other. the BB's have only one female so the hats having 3 would kinda throw off the battle match ups, since i think robin will fight ms devon and nami will battle lafite(since i think he is the 2nd most feminine BB pirate). that rules boa out, plus she has 2 many other responsibilities.
so looking at the ages i see someone joining from the age of 20-34. the strawhats have 4 DF users, so i dont see another fruit user joining even though a logia would be nice, but i sure oda will reserve those for future characters.
jinbei chances of joining have increased since he has left with luffy. jinbei is probably still the strongest fishman and since he no longer is a shichibukai the WG will be after him, so he will either join luffy or become captain of the sunny pirates again. since he is the strongest fishman i dont think he will leave his crew in someone elses hand.
i also think before the strawhats hit the new world, they will get their last member or members, so best bet is someone from fishman island.
oda did state he would show more of fishaman karate. a fishman karate user would fit in quite well with the other strawhats ability. everyone remembers him
http://onepiece.wikia.com/wiki/Kuroobi
he was weak but he still had some ok abilites. just imagine someone with his ability but a bit less than the monster trio in strength.
i have said a mouthful, but i final analysis is that a fishman karate user will join between the age of 20-34, who will be the helmsman of the crew, and his dream will be either, to be the helmsman for a great pirate crew, or to show that fishman karate is the greatest martial arts in the world, or show that fishman are equal to humans.
just like all the hats, he will be oppose to joining at first because the humans have oppresed the fishman, until he through a series of events he will have a change of heart.
also luffy will get his hat back before he heads to new world. also i think rayleigh will join the red hairs. he is to strong for luffy's crew and he cant stay on stay on SA since kizaru knows he is there. buggy will lead the the level 2 prisoners with mr 2 as his VC. crocodile and mr 1 will join the WB pirates.
THOUGHTS?