First of all, the usual hair.
The black cape must disappear. A cape will hinder Sanji's fast and graceful movements, both in terms of his fighting performance and the aesthetics of his battles. The fabric of the cape can be added to the suit itself.
The high and wide neck can stay, but I'd make it narrower and more closed, evoking the stealth of a trench coat with the collar raised.
The scarf falls now over the chest mimicking that of Zeff. Following this comparison, the suit's shirt is redesigned in order to look more like a chef's jacket (which kind of does already with those buttons and flaps).
The gloves stay, but in a more elegant way. The belt, number 3, headphones and all those Germa details can disappear. Personally I don't like the glasses nor the mask, but the latter may stay because of Soba Mask.
The shoes are redesigned so they look more stylish.
And that's basically it. Turn it into an elegant, chef-like black suit with red and white details.