I posted this on Reddit, but I figured it should go here too.
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Genderbending Sanji into a girl just does not work no matter what angle you take. Because as a friend of mine pointed out, Sanji's whole character is based on being a man.
1. If you make him a girl you have to also change Zeff, because Zeff won't employ or fight girls. While he may still adopt Sanji, he isn't going to teach him to fight if he's a girl, and won't employ him as he kicks his employees. So you either have to get rid of Zeff's chivalry (which you have to do anyway because there is no reverse form of it) and/or make Zeff a woman as well.
2. Germa, the Vinsmoke brothers were monozygotic quadruplets. While you could change them into multizygotic quadruplets, the expectations for Sanji vs his brothers changes. While Reiju was also modified, she wasn't outcasted for soft traits, because she was a girl and expected to have those. She still became a general like the rest, but from what we were shown Judge was not as hard on her. So therefore he probably wouldn't be locking a girl Sanji up in a dungeon, even if he thought he was a failure.
Now you could make Germa a Matriarchy, and Judge a Queen, but then the whole Sora thing gets dropped because even if Sora got turned into a man, the stopping the modifications plot is then trashed since a female Judge would be the one giving birth.
3. You can't trans him, because he completely rejected that in canon. And if you trans him as a kid before all that, he once more would not be trained by Zeff.
If you trans him, and make Zeff a woman, you still lose the chivalry and a third of Sanji's character.
4. Any route you take you lose Oda's message of 'Sanji is an icon for boys to know it's alright to cry, be soft, have fashion, etc without being reduced to being feminine for such things,' you lose the romantic side of him because he wouldn't have put women on a pedestal from childhood or Zeff, you lose the overly short-tempered thug side of him to men, he'd just be a reverse-Nami then going off on Women. You definitely lose the gag, even the lesser form of it, because most men would be fine being fawned over by women. You lose what makes him, him in various reactions with the other side characters.
Like, any way you try to rewrite Sanji into a girl loses half of his character and personality and writing, as his entire story arc relies on him being a man, Oda wrote him around being a man, and his backstory and personality ticks reflect this. Trying to make him anything but, changes his character and loses what makes everyone love him, turning him into another generic cookie-cutter metrosexual anime boy Twitter seems to love to copy paste the personalities of, or even worse an OC that only reflects the fan writing him rather than the character Oda made that everyone fell in love with.
Sanji changed, would not be the popular Sanji everyone knows and loves, as the reason he is so well balanced with the best writing is specifically all those things you want to ditch.
A trans friend on Discord of mine even had this to say (with his more erm non-appropriate language censored):
Thanks for reading on why Sanji was written to be a man, and revolves around being a man.