Here's an idea, why can't Oda look at other manga designs for female characters to get inspiration? Or meet other real women that he finds attractive and draw inspiration. Considering he has one of the most popular manga in the world I can't imagine that he would have too much trouble with that.
I don't buy the argument that he has to appeal to the teenage male audience. Like I said, he has one of the most popular manga in the world. One Piece is how many chapters in? Are people going to read 700 chapters just to get to the hot girls? (Or up to Enies Lobby just for fanservice?) If they made it through East Blue, I'm sure they are in for the long run. Besides, they survived Miss Monday, Dr. Kureha, Dadan, Kokoro, Lola, and soon big mom. I don't think that having a female character who is slightly less attractive than Nami is going to have them throw down their One Piece books and go read something else.
This is just personal preference, but I thought Nami was the most attractive during East Blue and up until Alabasta. But I prefer women with short hair (about chin or shoulder length.) Not that long haired women aren't attractive, I just don't like getting hair in my mouth or inhaling a tremendous amount of hair chemicals. But this isn't about me. What I'm saying is, it would be nice to have a variety of sexy women.
I thought Robin was much more attractive Pre time skip. I just don't like her post time skip look. Maybe I liked her bangs? Also I agree with some of the previous posters, her nose is awesome.
Anyway, there are enough attractive girls in One Piece that I can handle and average looking girl. I'd rather Oda focus more on substance anyway. (And yes a sexy woman can have substance, but I want her character to be more important than the look.) I would like variety because I'm bored with the current design.