We all know that Oda sure loves mythology. So since we’re going by mythology route, Sanji should have the third and final stage of Jambe which is Surtr Jambe.
In Norse mythology, Surtr is a fire demon or a primordial Jötunn (giant) present in the Norse mythology. He is the first fire giant who ruled in the fiery wilderness called Muspelheim. Surt was the guardian of Muspelheim. He stood at the border, brandishing a flaming sword. At the time of Ragnarok, the end of the world, Surt was destined to lead the fire giants into battle against the gods. He is also one of two “supreme evils” of the old Scandinavian myths, the other being Loki. Surtr’s name means “Black” or “The Swarthy One” in Old Norse. In Norse myths, the jötnar (plural for jötunn) are often referred to as “the opposite of the gods”.
Some highlighted connections here:
– Fire demon
– Loki connection (Both Sanji and Loki are princes)
– Surtr’s name means “black” and Sanji have a black epithet.
– The Opposite of the Gods (Sanji is/will be a D?)
Back in Thriller Bark, Zoro told Usopp that if Sanji keeps getting angry, he might transform into something. Against Queen, Sanji say that his fire comes from his passion so his flames could still increases even more (and we all know that Sanji’s passion is unparalleled).
Sanji said that due to his newly exoskeleton upgrade and his Armament that he has been training, his legs can finally handle the heat of the flames. Which means that Sanji could fight for a long period of time, it’s just that his legs won’t be able to handle the impact and heat.
And according to Islam mythology, Ifrit, also spelled as efreet, afrit, and afreet, is a powerful type of demon in Islamic mythology. The afarit are often associated with the underworld and also identified with the spirits of the dead, and have been compared to evil geniī loci in European culture.
I would add some small personal theory here, Diable Jambe would be orange/red colored flame, while Ifrit is actually blue. Then the third stage, Surtr, can be black flame. I think that would be symbolic considering how Oda pulls pun with “3” for Sanji. Sanji’s three stages, Sanji’s three flames colors, etc.
*Theory by Zoro D Goat