As one of the longest-running manga series on the market, One Piece has gone through many design shifts in art and style for its characters over the years. The most radical changes in character design, however, are from before the series was even released.
It's not uncommon for popular anime and manga series to release artbooks, most of which never see translation or release outside of Japan. These artbooks typically include reference sheets, style guides, insight from the animators or artists, and most interestingly, concept art from before the characters became as well defined as they are upon release. One Piece: Color Walk 1 is one of the rare artbooks that was released outside of Japan, receiving a translation by Viz in 2012, almost a decade after the book was first released. Some of the concept art included provides a peek at a very early Nami who couldn't be more different than her final design.
The concept art for Nami in Color Walk 1 depicts her with an absolutely massive battleax, giving her a real berserker-type vibe that strikes quite a contrast with the staff and weather magic she'd eventually come to be known for. The ax is also wrapped in a very heavy looking chain, and from blade to handle is nearly as tall as she is. Nami's hair and face are recognizably similar to her final design, although she appears to have pigtails rather than the short hair she possessed in her first appearances. Most interesting is her left arm and right leg, which appear to be mechanical or cybernetic, offering a plausible explanation for how this tiny, thin woman could wield such a massive weapon.
Although One Piece certainly came first, one can't help but compare the design to that of Fullmetal Alchemist's Edward Elric, who similarly has one leg and the opposite arm replaced with "automail," that universe's equivalent of artificial limbs. Nami's mechanical limbs are incidentally the exact opposite of Edward's, who had his right arm and left leg replaced. Edward's leg is also replaced above the knee, while Nami's appears to only replace the foot, although her striped sock could potentially be covering further enhancements. While Nami's final design didn't feature any prosthetics, the mechanical limbs did remain a canonical part of the One Piece universe, with the most notable user being Straw Hat crewmember Franky.
Another interesting thing to note is that Nami's shoulder tattoo is absent from these designs. This suggests that her backstory with Arlong hadn't been developed yet, either, and that this ax-wielding, cyborg Nami might have been a very different character than the classic Nami fans have come to know and love. What might the series have been like had this version of Nami gone forward? How would she have changed the direction of the story with such a wildly different appearance and backstory?
Insight into the development process for long-runners like One Piece is a good reminder that while these characters feel so crystallized now, there was a time before even Eiichiro Oda knew what he was creating, just like every other artist out there.