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, while the Straw Hats are running away from Big Mom in the forest, we see 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, in Wano. 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.

Posts by Gus
http://www.opfanpage.com/2018/01/24/oda-has-been-creating-a-whole-new-class-of-abilities-in-the-storyline/

感觉这个说的挺好的，所以搬过来让大家看看，不知道大家有没有讨论过这个问题

还有一个感觉说的更加有道理的。

Is Portgas D. Rouge the previous owner of Gomu Gomu no Mi?

It was stated that Ace’s mother held him for 20 months (5 months before and 15 months after Roger’s execution) in order to deceive the world of Ace’s connection to the Pirate King. Since Ace’s birthday is January 1, Roger must have been executed sometime between late September and early October.

Several theories claim that Rouge was the previous user of the Gomu Gomu no Mi and that stretching ability along with her haki is the reason why she was able to keep Ace in her womb for so long (as we all know thanks to Gomu Gomu no Mi’s powers Luffy can withstand large amount of distress and pain caused to his body and his internal organs).
But the reason she died is probably because for the 10 months she additionally held onto Ace, she was constantly ensuring Ace was protected. Whether she was constantly using Haki, or using awakened powers like Doflamingo did, I don’t know. But eventually even with the Gomu Gomu no Mi ‘s power 20 months were little out its capabilities and she lost her life!