Iceburg said that the only way to get Franky to join was by force. And considering that Franky's background is as boss of the Water 7 underworld, he is a "captain" in his own right, unlike any of the Straw Hats who has joined before.
Wouldn't it fit with his character to finish the fight he started earlier in the W7 arc, by starting chapter 437 with a Strong Right straight to Luffy's head? He probably knows he has no chance to win, but Franky has never been concerned about that kind of thing. I mean, since he's been a omnipotent leader for the best part of his life, he will probably not simply subordinate himself just like that. Not with him being so Super this week and all.
Franky would say that unless Luffy beats him, he won't join him. Luffy might humor him for a while before laying the law down, but probably way before the end of the chapter. Defeated, Franky says goodbye to Iceburg and his Family etc.
Also, the other thing that's keeping him indirectly from joining our favourite pirate is his obligation to defend the Franky Family. Perhaps right after (or instead of) being trashed by Luffy he will be defeated by Zanbai and his crew to prove their ability to fend for themselves.
That will definetly take care of the two last things holding him back from becoming a mugiwara, his pride as the strongest leader and sense of responsibility for his gang.