Since Black beard pirates have 10 members, i guess we can safely assume that the straw hats will eventually get a tenth member( unless Teach decides to kill one of is crew). Only then can the balance be restored.

Luffy got new members in east blue, grand line. the only place left is the new world. Therefore, the new member would be someone from the new world. Potential members include-

1) Jimbei- he has already stated that he would like to join them the next time he meets luffy( not directly though). Jimbei would be an invaluable member and no doubt he is as strong as Luffy if not stronger. His bounty is also huge. However, being a secondary character, his contribution to the storyline has been enormous. I think its safe to say that he will not be shown for a while. I also feel that he will be killed the next time he is brought into the story( thats my guess anyway).

2) Aokiji- He has retired his current status is unknown. Although it is highly unlikely that a former marine admiral would join a pirate crew, i base my assumption on this: the straw hat crew has paramecia and Zoan type fruit users. The only type of fruit missing is logia. My theory is that the next crew member will be a logia type and aokiji fits the bill. However, addition of an admiral class pirate to the straw hats would make then almost unbeatable and frankly mess up the balance between other pirates so not sure about this one.

3) Monet- Many people are saying that the next crew mate could be monet. she is also a logia type which supports my theory. Luffy would invariably ask her because he seems to invite weird people into his crew. But, Monet is a spy sent by Doflamingo(spelling??) so i doubt she would double cross someone like him. Even if she does, it is also highly likely that she will join Law's crew. I say this because she seems to have a soft spot for him.Since Law is also a shichibukai i doubt Doflamingo would try to attack her in any way. its a win-win for her. However if she is eternally loyal to doflamingo then she is out of the list( but we dont know that yet)

4) new character from next manga volume- possible but since the manga is nearing its 700th chapter, i dont think there would be any late additions. But if there is, i would guess its is a girl with zoan type powers( read it in an "interview with Oda" somewhere--- dont know if its true though)

seems legit??