A theory covering a potential secret form of Haki occasionally shown in the series.
Haki in One Piece was only revealed by the timeskip, halfway through the series. Until then it was an unexplained ability that some characters appeared to have. Once revealed, haki was mentioned to have three main forms, Conquerer’s, Armament, and Observation.
First of all we have Sanji who for some reason can produce fire out of nowhere and he can produce it anywhere, even underwater. Initially it seemed like this came from the friction from his spins, but by the timeskip it started to appear different. His fire seemed to appear from his passion. Similar to how haki comes from your willpower or spirit.
Luffy as well has used fire in the from of his Red Hawk. This fire might have something to do with Gear 2nd, but that is a little unlikely. It even appears underwater like Sanji’s does.
Finally, we have one more obscure instance, but it is still relevant. Zoro, when he was fighting Ryuma, managed to catch Ryuma on fire with his swords. The flame came out of nowhere and ended up finishing off Ryuma because zombies can’t be defeated through normal means. In that same arc we see a gag of Sanji starting on fire out of anger. That gag was later made a cannon ability of Sanji.
If this fire ability were a form of haki, then the monster trio would all still have it, even Zoro to an extent. Obviously, Sanji would specialise in it, but I think that would help ballance things out more.
The nature of this ability seems to be passion related. Luffy’s Red Hawk is in memory of Ace and sparks his passion. Sanji is a passionate cook, his fire comes naturally and is increased when he gets more passionate. Zoro’s dream is to become the number one Swordsman and while fighting Ryuman for his new sword, he became passionate.
I must also add that Kinemon can use fire attacks as well and Zoro seems like he would desire to learn that too, so in Wano, maybe, Kinemon could reveal how to use his fire technique and that it is a form of haki, perhaps known only to the closed off people of Wano.
All in all I think it is possible to have a secret form of haki. It would explain how some characters use fire and why the fire has such weird traits. The timing of it’s reveal also seems to work well with the current story line and may help Luffy and his crew defeat Kaido a little, so I think we could very well see this in the near future.
*Theory by Ma_D._Monk