Wow. S' a long time since I visited this forum… The following below is my two cents about the likelihood of the characters that have been discussed about joining the Straw Hat crew.
I'm typing this out under the assumption that the war between the Whitebeard pirates and the Marines will end in one of the worst possible outcomes: a total collapse of Whitebeard's reign (crew all but completely decimated, rival pirate factions trying to take over Whitebeard's territory, blahblahblah ).
Perona: For now, I feel that it's pretty given that she won't be joining. I'm kinda surprised as to why some folks think she will... Apparently the trigger being that she so very nearly became a stowaway on the Sunny. O.o The thing is, Perona doesn't seem to have any main motivation/drive for joining the SH. As a character, she doesn't act, she reacts. She runs away when it looks like Moria is losing or something ( my memory of the Thriller Bark arc is rather faint. ), she's just there in Thriller Bark as a villain...because. Yeah, because. XD Soooo... I think Perona joining the SH crew is quite unlikely unless Oda reveals some backstory/motivations for her. Perona might tag along with Zoro if he tries to get off the apparently deserted and godsaken island they're on, but only so she can make use of him to escape.
Boa Hancock: Possible, but unlikely. She has Kuja Island to worry about, plus throughout the time she had with Luffy, she has never broached the possibility of herself joining the SHs. It would be amusing to see her and some of the SHs butt heads though. I think Hancock's there to further expand the idea/existence of haki and explore the possibilities of having a shichibukai ally. Also as a plot device way for Luffy to reach Impel Down. Also! Possible future links to Jimbei: Hancock might be asking about that fisher tiger person or whomever who rescued herself and her sisters from slavery, since Jimbei himself is a fishman. Hmmm…
Jimbei: Another possible, but still unlikely. Taking into account the fact that there will/might soon be Complete and Utter Chaos happening due to Whitebeard's era being over, I can see Jimbei heading off to the New World to protect his brethren and/or keeping the peace between the fishmen, mermaids and humans. He might join Luffy for a while as their destinations might be similar ( Fishman island, that is...? ).
Ivankov: No. He's loyal to Dragon and his revolutionary army.
Crocodile: AHAHAHAHA. No. He's a self-serving kind of character. He'll only "join" Luffy's crew if there's something in it for him and even then only as long as it suits his motives/purposes. Unless Oda decides to pull a hundred-and-eighty in his personality but somehow or other I just can't see that happening. Croc would sell out the SHs in a blink if he thought he could benefit from it.
Ace: I keep swinging between "He's not going to join." and "He is going to join." -_- Um... It'll depend on what takes place in the chapters to come.
Oh Lord. It's all gonna "depend".
I can see almost all of the characters listed above joining Luffy as temporary traveling companions but as permanent crew members? Not really.
Seriously, we can talk about all of this until we're blue in the face but we'll only really get to know until this arc is close to over, or over. XD I may be making another post later on more general stuff about who the next crew member might be. Pardon if this post is a little incoherent. I'm kinda out of it right now.