Let me first make a disclaimer. The following text is purely My own thoughts and hypothesis. If you don't like it, don't hate Me for it. You are welcome to leave productive comments but please, no hating Me for this.
So My theory is that Nami is a member of the D.
First you might think that it's impossible for many reasons but hear me out.
Yes I know that she doesn't have the initial D. on her name but if you think about her past, it gives us a explanation. In chapter 77 Genzo tells Nami that Bellemere was in a war somewhere in a brink of death when Nojiko apeared in front of her carrying an infant. Nojiko tells that she's not her sister and Bellemere takes them both with her back to Cocoyashi Village. So we don't know anything about Namis real parents, nor where the war was fought.
Since Bellemere was a Marine, it's most likely that the war was initiated by the World Government. I think that World Government is capable of starting a war just becouse there is a D. on some island, if they find a good enough excuse. Remember Ohara. WG claimed the scholars wanted to resurrect the ancient weapons but the real reason was that they didn't want them to find out the truth about the Void Century.
Some of you might be yelling "She doesn't have the personality of the D. clan!" and that's true. BUT. What do we know about the female members of the D. clan? Only shown member is Portgas D. Rouge and we saw her only in a short flasback scenes during Sengokus speech in chapters 550 and 551. During those scenes only thing about her personality we are sown is that she's willing to risk her life for her loved ones. (Keeping Ace in her womb for twenty months) I think Nami has shown the same kind of persona when willing to fight opponents much stronger than her for the sake of her friends. So what I'm saying is that we don't know if female members of the D. clan share the same happy-go-lucky persona and the the lack of the fear of the death. Though to Me, it's kinda disturbing that in the scene about Namis past she is shown laughing in the middle of a warzone and piles of death bodies.
And then there is the fact that after the timeskip, Zoro gets some aspects that are similar to Rayleigh, most notably the scar on his eye. This clearly is a continum on all the comments in the series about how Luffy is similar to Roger. So am I the only one that think Nami looks after timeskip like Rouge?
I have been thinking about this for couple of years now and I just thought I share it with you. But as I said before, don't hate me for this.