The only real curve ball of a member that joined was Robin.
That doesn't prevent it from happening again.
I don't know, for the life of me, why people kept arguing against Brooke and Franky joining.
It was impossible to predict Franky would join until he captured Usopp to use as bait for Luffy. Only when he began to get friendly towards Usopp, and then was attacked by CP9 that he began to be a candidate.
I agree that Nami, Sanji, Chopper, Brook and Zoro were pretty obvious candidates, however Usopp seemed like an arc-specific character like Cobby, receiving a flashback and an invitation to the crew only at the very end of the arc. I know people who was surprised that Usopp joined the crew back then. Also, Robin's the big curveball, as you mentioned. Nothing prevents some unexpected character to join: any character may get motivations and past developed into a later date, giving him reason to join.
Feel free to quote me if I'm wrong, but discussions like this frustrate me.
Then why enter in a topic like this? Let people discuss their hopes and expectations. I like the see others' opinions, even if I don't agree with them.
Asking for Hancock to join is like asking Enel to join. Broken beyond all hell.
I don't see the similarity and don't think Hancock's power is broken. So far it has affected no powerful character: Luffy was imune, Momonga could resist it, and I don't see Whitebeard, Shanks or most of the Shichibukai. I can see Zoro resisting it too (Sanji, however…). We also don't know its weakness, but it probably has one way of reversing the stoning process.
The fact is: so far Hancock's power didn't do anything that a Gomu Gomu no (non-Jet) Gatling Gun, Oni Guiri or Robin's power (or even Nami's Clima Tact) can't do: clean up a lot of mooks at once.
If you are talking about her seduction abilities (which are not Fruit-based, since she could do it while using seastone cuffs), we saw that Luffy, Momonga, Ace, Jimbei and even Hannyabal did not fall for it (Magellan is another story...).
One could argue that she's too powerful because she's a shichibukai. I can agree with that, but there not all shichibukai are equal, and she seems to me to be the most inexperienced of them. All others were in the New World, while Hancock seems to have stayed with the Kuja attacking ships in the first half of the Grand Line (not a fact, just an impression I got. If she were in the New World, she'd probably have met lots of guys able to resist her power or use Haki and would not be so surprised with Luffy). Her bounty is also lower than any other shichibukai so far, and though bounties are not accurate measures of power, but it's intriguing why a character introduced so far in the series has a bounty lower than the Supernovas. Maybe Oda's making it so low so that she doesn't overshadow the rest of the crew, who knows?
Besides, the Strawhats are getting stronger. Could you have imagined Robin or Franky in the crew way back in East Blue? They'd be broken as hell. However, they are going to the New World, and will probably be stronger because of all their solo adventures, so Hancock may not seem so powerful anymore.
I know she can't join because she must protect her country, but there are ways the story can evolve that would allow her to go with Luffy (previously discussed in this topic, so I won't repeat myself). It depends on where this war's going and what's consequences we may expect of it.
I know the chances of Hancock joining are very slim so far, but she has such a quirky, unique and complex personality, unlike any other character introduced in this arc so far that made me really like her, so I hope she joins.
Of course, it's just my opinion, and this thread's about people exchanging opinions. Don't agree? Fine. I don't agree with Perona, Buggy or Hannybal joining, but I think people have the right to express their desires and wishes and put reasons why it could happen.