Let’s start with these panels:
When Luffy used Conqueror’s Haki in Udon Prison, we saw that japanese letter right in Luffy’s panel; the same thing happened when Zoro was about to use Asura to cut Kaido.
Now about Sanji, he also got the same japanese letter, which is hinted about Conqueror’s Haki, while he is in Bath House from Flower Capital when fighting X-Drake and Hawkins. Is that Conqueror’s Haki too? Maybe yes and maybe not.
Now, let’s see these panels:
Back in Sabaody Archipelago, we meet Silvers Rayleigh which is the Vice Captain/Right Hand Man of The Pirate King, Gol D. Roger
The first time he used Conqueror’s Haki, only Luffy, Zoro, Sanji, and Usopp got an highlighted/single panel about them reacting to Rayleigh’s Conqueror’s Haki. In that panels, Luffy, Zoro, Sanji aren’t even afraid and we even see Sanji’s eye looks like that.
One Piece Volume 100 Cover features Luffy, Zoro, Sanji, Ace, and Yamato.
Among these 5 characters there are 4 Conqueror’s Haki users. That means only and Sanji isn’t confirmed as Conqueror’s Haki user. If Sanji doesn’t have Conqueror’s Haki in future, why Oda willingly put him alongside these 4 of Conqueror’s Haki Users?
Also, Zoro and Sanji shared the same panel reaction to Luffy’s Conqueror’s Haki in Fish-Man Island:
Sanji is definitely going to be a top tier fighter at the end of the series. Conquerors often reach positions of higher status or regard and we now know that he will eventually be widely regarded as one of the Wings of the Pirate King. Strength is also a factor.
Zoro and Sanji being Luffy’s wings mean that Sanji will have to have the ability to fight with the strongest on the seas. He will probably need Conqueror’s Haki to do that and would be much easier to write if he had it now that we know how it works.
I think Sanji will end up having Conqueror’s Haki too, Oda gave us so freaking many hints about that!