What I wouldn't want is a fishman/mermaid to join. There are enough mermai/fishmen in OP world. Stay tune for the next arc, I expect atleast 40 named sea creatures to be introduced there.
As for the feminine matters, Robin represents mature beauty and Nami represents cuteness. I don't want a useless emo teenage mermaid whose dream is to find her lost memory or a kid with amazing powers of Apis' age and Chimney's size to join the crew. I would prefer a woman in her fifties and up, not a Biscuit or the latest Hokage(forgot her name) type though who got an Age Age Devil Fruit . I don't care about her powers, she could have an alien race or whatsoever. Plus, I heard Oda wants to draw a manga about old women. Or better yet, no more female female members.
Based from Wanted, the main character's job that are featured there are pirate, pickpocket, samurai, gunslinger/cowboy and a monk. The first four I mentioned already mirrors the first four Mugiwaras so a monk would be likely. Sanji doesn't sound monkish. Maybe a cowboy themed should join but he doesn't really need to use guns though. I would love to see a Spaghetti Western themed Strawhat. I doubt Usopp counts, or Robin wearing a Cowboy hat.
Also, we know Oda loves movies. Do you think Oda based each Strawhat from a film genre?
Luffy - Adventure/Children's
Zoro - Action
Sanji - Romance
Usopp - Comedy
Chopper - Animation/Fantasy
Franky - Sci-fi
Robin - Mystery/Noir
Nami - Porn
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Brook - Horror/Thriller