Something Sanjibros above forget:
Design
- He has black-colored goatee, so black hair Sanji is possible to exist.
- Luffy has a mode where his hair turn from black to white, Sanji doing similar hair color's change is possible.
Personality
- Some Sanjibros want him to be able to hit women if there's Black/Evil Sanji.
- Some Sanjibros want him once again to have another focus & subplot from Oda, thus cementing him as 1st/2nd most well-written character.
Abilities
- There are 2 Germa's abilities Sanji has yet to get/refuse to have : the Raid Suit and the invisibility that comes with it. Black/Evil Sanji means he won't mind having those, thus boosting Sanji to one tier higher again & possibly barely reach Emperor tier.
Last but not least, Black/Evil Sanji means he has domineering, aggressive, tough persona, which fits the profile of CoC users. Some Sanjibros that want him to have CoC also subscribe to this theory.