I agree with Mugiwara-Kun on a few points,
Noone could predict if Cariboo will join or not… but like I said before @ the Spoiler thread, No one knows. His ambition is to kill the strawhats in order to get a famous name or w/e else.
He is a Logia for sure, we saw this on the first ep he used his powers, but like someone said in the past pages, he is able to control his logia powers, and got hit by surprise in a funny contexte. he wasn't trying to run or fight against Sanji nor FranKy.
and he is attending the perfect timing to try and kill Luffy.
We still don't know if he was serious when he was beggin luffy to back off because of that Kraken or was just tryin to pretend hes too weak to cause trouble.
But we know that hes very agressive and that he got a famous name & bounty for killing the marines ? and Noone of the current strawhats is the kind of pirates that kills or bury alive marines thats Totally insane, I don't except this at all! They're pirates, but they're good people.
So, if you get me, you'd prolly understand that Cariboo is like Eustace Kidd, or maybe his subordinate. who knows, this is one piece.
annnd, if Cariboo wasn't really playin the role of the afraid guy and was serious, then I think the fact to see Luffy using King's Haki to tame that little guy as he called it, he will prolly back off and get down on his knees or w/e. He would make a good SH, just like Squardoo & WB.
(But nothing is sure ya know, He got a very bad reputation for killin innocent people, I don't think that he'd get a chance to join)
so lets just see what oda plan for the next² week.