When Franky joined the crew, it was a gradual integration process of Usopp moving him to tears, him getting involved in the whole Nico Robin arc because of his own past and subsequently siding with the strawhats when it mattered.
Perona has a few problems to handle before she'd become a candidate for joining the crew. First she's currently obsessed with learning what happened to Moria, she's therefore still very loyal to him. She's still a subordinate, even though Moria's dead. Moreover, when she learns that Doflamingo got Moria, she'll want revenge on him for sure, and she's in fact one of the better matches to deal with Doflamingo's ability to boot. And most importantly, she currently only got attached to Zoro because she was lonely, but she still hates Usopp's gut for what he did to her and her animal companions. She has other priorities. She will surely tag along with Zoro to uncover the truth about Moria, but more must be resolved before she'd be a candidate to joining the crew.
Plus, she has a characteristic "custom evil laughter" that Oda's biggest villains all have (Arlong, Crocodile, Enel, Moria, Blackbeard off the top of my head). *Foreshadowing*
(Not to be confused with the custom heartfelt laughter that all giants to date have)
Actually this also makes me realize something interesting too. Franky and Brook were both leaders of a team. Zoro, Nami, Usopp, Sanji, Robin were all in a position where they led people at some point or another(Zoro's his bounty hunter friends, Nami is the village's security she took charge of, Usopp was his Usopp pirates(fitting for him), Sanji was assistant to the head chef, Robin was Miss Allsunday in BW) Chopper is the only character who had never really played a leader role before joining the crew, and right now he's pretty much leading mediation between 2 groups. It's a very subtle trend, but if anything it suggests that the crew is a crew of leaders of some sort, and therefore the 10th member should also have been influential at some point to follow that trend.
This would make Perona kind of valid(she was a vice-commander of the Moria pirates) but it doesn't discount Jimbei or Hancock one bit. I'm getting really curious now.

(Random tangent: If we're mentioning Perona maybe we should mention HERACLES'N! ...but let's not XD)