Sanji does have an immense will, but is that enough to get conqueror Haki? He would need to deal with his issues and stop giving up on himself when he's in very difficult predicaments first. Then and only then, he could maybe awaken CoC. If it happens, in my mind it would be end of story in his final battle. What sort of king Sanji would be though, not king of cooks, nor king of women, of perverts or whatever, nor king as royalty of kingdom. It would need to be something meaningful. King of love or kindness sounds lame, perhaps the king of justice or simply the king of flames burning hotter than anyone even Akainu. I don't really see the need for Sanji to have conqueror though other than the monster trio and Sanji vs Zoro agenda. It's going to be funny when Usopp has conqueror Haki in Elbaf and Sanji doesn't.