We’ve seen Haki and Devil Fruit abilities but there’s another class of abilities Oda’s been sneaking into the storyline that he hasn’t officially announced. That is Elemental Abilities that are not tied to devil fruits.
The hints start back in Fishman Island after the timeskip where Brook learns to use ice elemental with his Soul Solid ability. He’s able to literally freeze enemies with his sword attacks.
In Punk Hazard, Zoro also noticed Kinemon can cut with fire. This ability seems to surprise Zoro in that he didn’t think it was possible.
Fast forward to Whole Cake Island and we see Big Mom’s Prometheus and Zeus are elemental attack beings and were injured by Brook because he was able to use elemental.
In fact they were shocked that it was possible for them to even be hurt. More recently we saw Luffy attempt to attack Prometheus using Haki but it has no effect. Jinbe explains that they are elemental and Haki has no effect on them; one must fight elemental with elemental. We also see that Luffy’s Red Hawk ability literally becomes fire elemental when he uses it, being able to melt candy.
I think by now it’s clear that Oda has been creating a new class of abilities in the storyline- Elemental. I suspect this class will show itself more predominantly, and perhaps even given an official name. We know Zoro and Brook had great interests when they saw Kinemon’s fire-sword style. It’s possible they may learn new elemental styles from their time in Wano. I also feel like elemental powers are a good way to explain the powers of Shank’s crew; given they are likely to not have any devil fruits, it would be weird if the whole crew just had Haki with no other outstanding abilities.
*Theory by miyabiz