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These are the resons why i believe Shirahoshi will be:

First of all, she is a woman

A mermaid

She met Luffy and Luffy helped her get out to the sea forest

The stra-hats don't discriminate anything

She was kept in that tower for 10 years, anyone would want to explore the world, maybe she'll take on her mother's dream and after Luffy saves Ryuugo from destruction, the fishmen will gain more trust in humans

If that happens than the fishmen wont dicriminate humans anymore and she would need to spread her mothers message to the human world. What better way would there be if she'd join a crew that will reach "the end of the world"

Another reason would be because she is weak, think about it, each of the straw-hats trained hard these 2 years and they all became strong, but if someone strong would join the crew, the bond wouldn't be that strong as when someone that is saved and protected by them

Even if she is a princess she isn't like Vivi, she can't do very much in Ryuugo, she isn't the next for the throne as she has three older brothers, also she isn't necesarily bound to her land, she doesen't seem to love her country as much as Vivi, but she loves her mother more.

Every one of the crewmembers had a flashback and one of their important people died or left(they left or were left) and sanji's flashback was about how he met his important person and how they barely survived.

Jinbei could also be the one joining as he is the one having the flashback, Fisher Tiger died, he already knows and has somewhat of a duty to protect him.

But I believe Jinbei has the duty to protect the kingdom(he is also a fishMAN)........... AHHH! and i wanted to say one more thing but i forgot, damn it, well than tell me your opinion about what i said.