Zoro already has it imo.
He made Urashima run away due to his weird aura
He glared at Enma and made it give him back his haki
etc
Zoro may unlock it, but sure he doesn't have it
Otherwise, Zoro wouldn't on many occasions when they have no time to fight fodders like in Dressrosa when whole city is after them or in Wano when they were looking for Otama, Zoro if he has it he wouldn't tell Luffy they don't have time to waste on them so that Luffy knocks them out with his haki. Those were the biggest signs for me that he doesn't have it, cause if he had it, he wouldn't himt at Luffy to use it, he would have done it by himself
Also, regarding Fishman island quote from Zoro, the translation varies from one version to another, sure one version felt like he has it where he stated about Luffy using king's haki "if he can't do that much, he would have to step down as a captain" which sure for some fans sounded like Zoro can do it
But other translation versions says "if he can't do this much, we would have to find a new captain"
meaning trying finding someone with quality of a king, someone ELSE, new character as captain (Zoro himself is NOT included)
I first was hesitated if he already has it, but since he relies on Luffy for using it multiple times, then he shouldn't have it yet.
Though I do agree that he will unlock it. There is absolutely no way I will accept that he doesn't have the potential.
Sanji on the other hand, although the guy is my favorite character, I will say this, if he EVER had it, it would be related to royalty bloodline (since Oda stated in databooks before that previeous kings in the past used to use king's haki power)
However, looking at scummy Vinsmokes, I have hard time seeing that LOL, but who knows.
And I will admit that, Sanji DOES NOT have a strong determination and well as Zoro or Luffy, making it harder for me to see him unlock it.
I have yet to see Sanji in a scene where all odds are against him, and he still plans to win.
I feel that's even the only thing that makes Zoro show more feat than Sanji, is because he pushes himself beyond his limit while Sanji gives up easier, which is why I feel he doesn't have a quality of a king to use it.
Sanji only competes to the last breathe with Zoro, that's the only time Sanji endures as much as Zoro. but other than that for their rivalry, Sanji does not try to endure as much when he's facing other obstacles.
He rather gives up easier which is a shame, but it also reflects his character which takes the decision based on rationality and reason unlike Zoro and Luffy who never looked at reason or rationality in matters of winning or losing.
Sanji's case is somewhat similar to Law, Law as well is very much like Sanji in that regard. While Kidd is like Luffy.
Look at Law vs Mingo, and Luffy vs Mingo as example.
Law after Gamma Knife: He lost hope, and didn't want to continue, he was screaming "damn it, damn it" since he didn't finish Doffy yet.
Luffy after gear 4th ran out, Hhe was trying to escape, his focus was solely focused on "I have to win 10 min to finish him with ome final blow"
HUUUGE difference in mentality.
I would say that's sadly the negative side to strategic rational characters like Sanji and Law, is that they hold back their progress with haki because their determination is backed up by rational and reason, where they give up easier than the likes of Luffy, Zoro and most likely kidd as well
If Sanji ever changed that attitude of his, and gained the "never give up" attitude, Sanji imo would close the very small gap between him and Zoro which imo is the only reason for Zoro's slight advantage over him in combat