I vote for Nami. She's pretty weak right now, and it would complement her skills. Plus, she could grow up a bit if she trained to make big fire and weather.
Luffy would make sense too, if his haki lets him stop the bad effects of two DF. That's plausible, since the conqueror's disposition negates DF powers. Maybe he can target it on himself, permanently. In other words, he would just be that bad ass.