I got a better one
Diable Jambe and Ashura and KHO
The bases of sanji's abilities needs some elaboration and so does Ashura
Asura and Jambe are one of those stranger abilities that really need some kind of further elaboration. Even during Ennies Lobby, we got a decent explanation for how Luffy makes Gear 2nd work by using his elasticity to pump his own blood faster, and it even got some nice foreshadowing in how he got the idea for the technique in Movie 7 even if it wasn't in the manga which it should've been.
All we get for Asura is Kaku just saying that Zoro creates it through "sheer will", and before that Zoro even made a giant ghostly image of a monster appear when they first entered Ennies Lobby as a different attack. I mean, haki is said to be spiritual energy itself, yet Zoro was seemingly manifesting his spiritual energy way before anybody else in the series and using it to make semi-solid constructs capable of deadly force which no other haki masters in OP have been able to do, and using it in a far cooler way than any other bs haki technique Oda's introduced so far, and yet you're telling me Zoro needed to perfect his haki through training with Mihawk? And no other great haki hack user can do the same? Come on now. Unless the whole thing is an illusionary kind of haki and Zoro just increases his own enhanced speed to make himself cut 9 times faster, but it still doesn't explain why no other haki guy can make demon and ghost projections for attacks.
And for Diable/Ifrit Jambe I thought it was Sanji using friction to heat up his leg and light it on fire and maybe using his trained durability with Zeff and later haki to shield his skin from getting burned, but ever since the timeskip Sanji can just light his whole body on fire without any kind of friction, and even his hair did this strange little devil horns thing, and during Zou it looks like Sanji may be pyrokinetic since his cigarette just burst into flames when he got angry, and during Wano he lit his foot on fire with no shoes or pants, just a sandal on with no haki coating whatsoever and yet the sandal remains unaffected by the fire. Like is the fire partly spiritual or illusionary like Asura or can he actually just control the fire's temperature and intensity freely? Or did he just coat only the sandal's strap in haki? And what's with Ifrit's blue lightning? Or is it all repressed side effect of his father's experiments or something from Sora or Zeff we don't know about? There's a lot that I wish Oda was willing to answer on the matter.
We also still haven't gotten an explanation for Nami's borderline supernatural sensory of weather, like if its observation haki or something else, and it hasn't really been acknowledged much in ages.
