Brooke seems most likely but I'll wait until I see if he fights first. However, if Perona joins, I'll go with that too. Don't see how different from Nami she'd be but then again, we know next to nothing about her right now.
And yes I believe another female is needed. Not for the fanservice, good lord no. Just to get some more variety in my pirate crew. Heck, I wouldn't mind if the Cerberus somehow became a new pet for the Mugiwara Kaizoku. He seemed to fit in with Luffy and crew while he was walking around with them.
Hmmm, now that I think about it, what if Perona could use her ghosts to control Brooke like a puppet or something to that effect? That way, both Brooke and Perona could join and work as sorta of a two person combined into one unit like a tag team or something. (Think Ice Climbers in Smash Bros.)