I'm hoping it's not Haki. If only because I find the idea a little boring. But, the "Haki Lightning" we see coming off Scopper's axes does kind of point to it being some form of Haki.
Maybe "Concentrated" Haki? Maybe instead of coating your entire arm or body with Haki, you have to concentrate the totality of your strength into a single point? Doing it the way that Luffy's been doing it so far may actually be wasteful. He might be using up power that he doesn't need to.
"The Power of Love"...is just another form of Haki, to me. At least for now. And it's one that might be kind of hard for someone to teach? I don't know, I guess we could spend a couple chapters showing the Strawhats unlocking their "loves". Sanji's overflowing love for every woman around him. Zoro's love for Kuina. Usopp's love for Kaya. Luffy's love...for meat? A true "Romance Dawn"! But...that would require Oda to get sappy. And Oda's said before that he's not confident in his ability to write a romance as a focus. So, I'm personally doubting this will be the key. I don't think Oda will go this way in a action manga.
Seastone Weapons...feels unlikely, at this point. I don't know. If Oda was going to introduce a new way to hurt Abyss users that wasn't Haki, I feel like he'd make it more interesting than just Seastone. If Oda's going to do something different, I feel like he'd do something more creative. If this was going to happen, it probably should have happened on Wano. Since Wano was where most Seastone was being mined.
I do like the idea of it being tied to Devil Fruits in some way. That way, we can get a "Weapons Triangle" going with Haki beating Devil Fruits, Devil Fruits beating Abyss powers, and Abyss Powers beating Haki. Abyss Users can just regenerate from a Haki-coated punch, after all. I do like the idea of Scopper's axes having Devil Fruits, like Shamrock's Cerberus Sword. Weaponizing the destructive power that's created by having conflicting Devil Fruit "wishes" around each other. But, we haven't seen ANY sign of that. So, this is probably a non-starter, too.
There is a possibility that the only way to defeat Abyss Users...is to be a Abyss User yourself. Maybe Scopper has a Abyss Mark Tattoo somewhere on his body? Maybe Chopper will see it while he's treating his wounds? The black "Haki Lightning" could actually be "Abyss Energy". That would make for an interesting twist. And a interesting risk/reward choice to make. You might be able to get the power to defeat these guys, but Imu's eyes might be watching you all the time from now on.
And I do wonder if the Abyss powers are more broad and have had more foreshadowing than we might think. Because we HAVE seen something VERY similar to Abyss Mark powers before.
Miss Goldenweek aka Marianne's Color's Trap.
https://onepiece.fandom.com/wiki/Colors_Trap
The Abyss powers seem to require you to put a mark somewhere on your body, which alters it. Color Trap alters your personality, after you get "marked" with a symbol. Both seem to require putting a symbol on your body, in order for the power to take affect. So, maybe Miss Goldenweek's Color Trap is another version of the Abyss Mark? These two powers might be part of the same larger power system. Oda MIGHT have been planting the seeds for the Abyss Mark years ago, and no one's noticed yet.
Also, the Colors Trap is called a hypnotic ability. It's a "mind power". And...well, I've already speculated that the Abyss powers could actually be the "Mind" power to compliment the trinity of Haki's "Body" and Devil Fruit's "Soul" powers. So, this kind of fits for me, personally. Both powers require you to have "knowledge" of what symbol to use and how to use it. It's a more "mental" power than the other power systems we have in the story so far.
https://worstgen.alwaysdata.net/forum/threads/haki-and-devil-fruits.66831/#post-6049404
So, maybe the Strawhats will be getting some "war paint" in the near future?