The shonen genre has its preferences for boy/man characters over girls, but I don't think that's a hard rule, and things are evolving. Even more so because nowadays there are plenty of girls in Japan reading Weekly Shonen Jump, and also the meta in anime has developed into a bunch of cute girls doing all kinds of stuff (trivial or badass). The audience in the genre is claiming for more and better girls.
And I don't see One Piece being overly conservative in the matter. On the contrary, it is above many other series for having written many fascinating female characters (including Nami and Robin), even tough there's an obvious sexism in the manga. Unfortunatelly, it could be much better.
However, we don't need to be pessimistic about the future female characters. Oda shows talent writing them just as much as the boys. Even design-wise, for each Nami-face there are some girls with different looks and body shapes (although the boobs are probably gonna be big).
But people should know what to expect from a female protagonist when it comes to design, because authors have to pander to the tastes of the audience (and this narrows the options). Girls will be pretty, relatable and somewhat standard. To be fair, this also applies to men too!!, although it's easier (very common) to break the mold with supporting male leads (like Franky), for a lot of different reasons (meta and social), but never the main character (like Luffy). You see, even outside of animation things are like that.
Whatever kind of girl you want to see for the next Strawhat, just adjust your hope enough to include a pretty face (and you know exactly how Oda draws pretty faces) and a behavior that fits. That's a little depressing, sure, but at least the girl can still be cool and original (like Carrot).
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