We already know that Usopp is Yasopp's son, Robin is a child of Ohara, Chopper is a reindeer, Luffy is Dragon's dad, etc etc...
But what about Sanji, Zoro, Franky and Nami?? Where did they come from and who were possibly their parents?? Other than some tidbits like Franky was a son of pirates, Sanji was an apprentice cook, Zoro learnt in a dojo and Nami was picked up from a war, further details are still blurry.
Even though I think Oda will reveal more about them in the future, I think it would be fun to speculate about this. So this thread should not be taken as canon, just a place for some sparkling imagination and fanfiction <<what the hell am I trying to say here? >> .
Well, if you ask me, Franky may be a son of some great pirate or a crew of a great pirate (like Usopp), Zoro is an orphan, Nami is a fallen Marine's daughter and Sanji is an A.L.I.E.N (the curly eyebrow is sure freaky...)