His epithet will probably be either 情熱の王, Jōnetsu no Ō, King of Passion, since “King of Love” sounds weird as hell, or 魔王, Maō, Demon King.
Maō would fit his imagery especially well. His family are basically portrayed as moral demons, and his two main modes are Diable Jambe and Ifrit Jambe, both named after demonic beings. The natural next step could be Sanji becoming the king of all demons.