Luffy has a level of willingness to allow change and accept it, he accept Franky despite what he did with Usopp because of the events of Enies Lobby, as with Nami and Arlong Park, same Robin and BW/CP9.
I honestly don't see her joining but the more people talk about it the more I think of the possiblities of how she can join. I wish people would stop giving me stuff to think about, my brain is getting bored and now is beginning to want her to join. Basically:
-If she joins she'll need a new dream
-Her power is pretty impressive, on the level of Robin
-She has quite a unqie design, even in comparison of some of the other past villians, only bested by designs like Mihawk and Crododile… The really high-ups. She got more character build then Hogback and Abs-Invisible guy, thats about it.
-She is positive despite her powers.
-Her personnelity overall is a step above most characters.
-There are aspects of her personnelity which the rest of the crew doesn't have but she has, a little more balancing within the crew would result from her joining.
Reasons against:
-No background (though you see her as a child in Hogback's flashback just barely)
-No dream that can be achieved without Moria
-She and Usopp won't get along, maybe she can still work her way around Robin, Nami, Chopper but not Luffy, Sanji, Franky, Zoro because they got hit by her powers.
-The only thing I can see her becoming attached to that would keep her with the crew at the moment is Chopper (cute with cute voice).
-Not a consideration is in her head right now about switching teams.
-Only one case so far of two people joining (Franky and Usopp).
-She'll be in the bottom tier along with Nami, Usopp, Chopper if she joins due to her powers weakness leaves her very vunerable.
Honestly... I can only see her joining the SHs to either to save herself being kicked off their ship when they come back or because she just wants to be on the strongest side for safety reasons now Moria is gone. She is very afriad of them right now because of Oz's rampage is bring out paranoia in her. There are other scerenios, but Oda would have to be really good in order to pull them off. I wouldn't see her as true joining either, she would most likely end up on per with Usopp (who had been playing make-believe until Enies Lobby), Nami (who was an ally at first and then betrayed them for Arlong and then later joined) and Robin (who had expected to be forced to leave).
She may help the SHs if she can't run away in the TS because right now she believes Oz is after her and doesn't realise she is after Nami.