In my opinion Sanji's final upgrade should be Raiden Jambe and he should have electric powers

>already had lightning in Ifrit Jambe
>can say he copied lightning from Yamato the same way Zoro copied flame rend from Kinnemon, Yamato and Kaido used Thunder Bagua to blitz for super human speeds
>perfect for Sanji because it can increase his speed to the peak and allow him to match Kizaru's speed
>ties into cooking because electric cooking
>can use electric observation since his strongest haki is observation, just like how Enel used lightning for super human observation
>could use transmutation to make his skin armor even harder mixed with his observation, just like Enel used lightning to transmute his spear
>in a lot of media lighting is considered the advanced form of fire

I'm sick of all these "JUST HAKI HARDER" or flame based powerups