I dont know who will join the crew yet from this arc (and there wil be someone… I have yet to see Oda tantalize people with a new position on the crew only to have it snuffed away from them after the end of the arc.) And by position I mean things like doctor being needed (chopper), shipwright (Franky), cook (Sanji), and even Robin had alittle exposure as it became more and more clear that the straw hats needed someone who had a grand understanding of the world... it just came at them faster then they realized they needed it.
Brooke is the likely candidate well... because other then a tragic/personal past and a dream, he has all the makings to be one... but I also want to bring up paulie as well... sure the main difference is that Brooke was asked and accepted (unlike Paulie), but so far Brooke has shown no attachment to the crew other then him saying "yay I found humans who like me" (Though that can be remedied easily with a plot chapter or two devoted to Brooke.) and has not made him stand out other then that. Remember Luffy wants nothing but the best or the types that aspire to be the best in whatever there profession or dreams are. He says he is a "decent" or great musician on his old crew... but does he aspire to be the best? thats the real question.
With that in mind, I am honestly leaning towards the upset in Cindry, mainly because I dont know why Oda would flash us with news of cindry of her being an incredible entertainer in her past life if it didnt mean something. Also Entertainers are known for a wide variety of talents including being musicians. So you get the best of everything, but since she was supposedly killed in her prime... no one knows how far she could have gone. That alone sets the stage for a potential sad past with a dream hidden behind it. And im not talking about the plate dream, that seems more like a mechnism to hide a hidden guilt (and also a reference for a famous bed time story).
Also the one thing I have learned in One Piece is each of the main characters has a style of greeting. Such as sanji's "shitty" remarks. But other then Robin (and she only laughs this way when she is really happy.. the Dereshishi laugh), only villans had distinguishable (I mudered that spelling...) laughs, or one shot characters from arcs. Brookes Yohohoho laugh makes him stand out along those same lines which further makes me question whether he will Join the strawhats... but then again we are really early in the arc, who knows what might happen by the end.
Personally I am going to wait for any further observation or guessing games till we get another 10 chapters into this arc. By then it should become pleasantly clear the direction Oda wants the end to become as he always seems to develop the plot to meet an end, rather then surprise us with an upset.