Perona's: to have a country filled with cute servants and do nothing for ever.
Yeah.. that's not a main character dream, at least not a young one.
Hancock's: to never be controlled ever again (except by her own heart) and to make Luffy happy.
Perfectly good reasons to join, if this was a romantic love oriented or romatic love even EXISTED in the setting.
Jimbei's: to keep fishman island and it's inhabitants safe, to honnor WB and Ace's last wishes and protect Luffy.
He's on a bit of a bind there, either he goes on with the dream of someone else, or stays for the safety of his nation. Either way I don't like these dreams.