Coming to the point, we all know that Sanji had a secret past and much of it was foreshadowed. However, I think it’s not the only thing that’s a secret about him. I believe that Sanji has a secret power since a very long time ago. Someone on Youtube (I forgot who it was, I watched it a long time ago) made a theory behind the reason why the Vinsmokes kids have curly eyebrows and the reason given was that the Vinsmokes kids were genetically modified to have devil fruit powers without losing their swimming abilities (or something along that line). It has also been mentioned that Sanji was the only one who did not show any signs of having any abilities (exoskeleton…) and that was the reason he was rejected by the other members of his family except his mother and Reiju.
Some high-ranked officers of Judge thought that their might have been a delay in his “awakening”, but nothing ever happened and he was locked into prison. I personally think that Sanji doesn’t have the exoskeleton of the Vinsmokes, but he does have a weaker form of their other capabilities. I also think that Sanji awakened his power when he started to learn how to fight, which was with Zeff, and that he was rather young and therefore unable to control it yet. When he uses Diable Jambe for the 1st time against Jabra, he says that “I’m the type that gets heated up by anger.”
And therefore, to have this power always under his control, he would have to stay calm at most times, which is he’s usual manner. I think that’s what led him to smoking. I’m saying this because every time that Sanji seems pissed about something you’ll just see him take out a cigarette and start smoking.
Now, going to Sanji’s powers. Obviously, it’s linked to his Diable Jambe. We know all the Vinsmokes alias and some of which, we know the powers. Reiju, “Poison Pink”, has a penchant for poisons and has powers linked to poison. “Electric Blue” Niji has the power to control electricity. We don’t yet know the power of Yonji (“Winch Green” which could have something to do with plants), but we know that he’s the weakest, and Ichiji “Sparking Red”, I believe, has the power of explosion, for in chapter “1 and 2”, where is introduced both Ichiji and Niji, we can see the city burning, and I believe it is a result of Ichiji’s powers and not some cannons of Germa 66.
So, they all have a certain power associated with a colour and Sanji’s could possibly have been “Flaming Yellow” if he had stayed with Germa. Through Niji, we also see that their powers can be used by any parts of their body as he uses it in his hands. Therefore, we can deduce that Sanji’s Hell Memories is a “full cowl” version of his Diable Jambe, but he uses only his legs, so it does not help much. Also, the first time Sanji used Diable Jambe, he was aware of him having that power, unike Zoro with Asura. We can therefore assume that Sanji knew about this power, but hid it, maybe because it was dangerous when he wasn’t able to control it.
After the timeskip, Sanji can use CP9’s Moonwalk, calling it Skywalk and I was thinking that maybe it is not the only thing we learnt from CP9. Another technique using feet in CP9 was Ranyaku. I believe that Sanji has also learnt this technique or will figure out how to use it soon enough (because, seriously, I think that he is quite underrated, even after the timeskip), but will also add his own twist to it by using it with Diable Jambe/Hell memories, therefore having flaming Ranyakus. I also think that Sanji’s epithet, Black Leg, is a foreshadowing of his future technique. During Big Mom arc, when the StrawHats meet Pudding, she even asks them if Sanji’s legs are black, as his epithet suggests, but Nami says that they are not (the epithet probably comes from the fact that Sanji always uses his legs and that he most frequently wears a black suit, thus emphasizing on his black legs). When Sanji fights Judge, he is surprised by the fact the previously weak Sanji is able to use Haki.
I, therefore, think that Sanji is going to use haki more frequently in the future, which will result in him coating his legs with the armament (usually depicted as black), and even reaching the stage that he will be able to use Diable Jambe while coating his flames with haki, thus the black legs, and maybe even combining it with the Ranyaku and having “flying flaming haki-coated kick-slashes” … or not.
*Theory by natrium23