Ok, so here me out, because I have an off the wall speculation that's about to transpire, but it fits so perfectly.
So this recent chapter we have learned that there is a figure that has appeared in front of Zoro, that looks exactly like the Grim Reaper. Suspiciously enough, this figure also happens to look exactly like Brook. Now currently, it doesn't make any sense as to why Brook would be standing in front of Zoro, with a giant Scythe, looking like he is ready to attack him. But what if there is a very plausible explanation to all of this? For that, we are going to have to touch upon a few other plot points.
So lets head back to the very last chapter (Chapter 1037). At the very end of the chapter, we have the Gorosei discussing a specific Devil Fruit that hasn't awakened in centuries. There has been many, many different theories as to what Devil Fruit this could be, but I think Oda gave us the clue at the very end of Chapter 1037 as well. While the Gorosei is discussing the prospect of this Devil Fruit awakening, we get a huge panel of Zunisha appearing off the Shores of Wano. One thing that this scene is reminiscent of, is the huge shadows that we saw on Thriller Bark.
I believe that the Gorosei discussing this specific Devil Fruit, in tandem with a scene reminiscent of Thriller Bark, was a clue that we had to head back to Thriller Bark to find the answer, and man did the answer come. So lets go back to Thriller Bark, shall, to a very specific chapter, discussing a very specific detail that we all have forgotten about. And that is chapter 443, where Brook is explaining his Devil Fruit.
Now in this chapter Brook explains that his Devil Fruit serves one purpose. To revive the user from death, only once. Now on the surface, that's an extremely lackluster ability, to where even Brook said that while he was alive, the only thing the Devil Fruit did, was make him unable to swim.
Seems like a pretty shitty fruit if you ask me, with a terrible payoff. You revive once, can't swim, and that's it. Even after 50 years alone, while on Thriller Bark, Brook still never managed to figure out anything special about his Devil Fruit. The fruit was named the "Revive Revive" fruit, and up until that point, it's only purpose was thought to be bringing the user back to life once. This all changes however, since post timeskip, we've learned that Brook has slowly been starting to be able to do more and more things with his Devil Fruit, that him, nor us, nor the people of the One Piece World thought was possible. This is completely inline with what the Gorosei were saying about this mysterious Devil Fruit that hasn't "Awakened in decades", because even though Brook had the fruit for 50 years, he still couldn't do shit with it, other than revive once.
This also fits inline with the naming of the fruit. The "revive revive" fruit, and the user can only revive once. If it's true name is hidden, then nobody is going to think that there is more abilities the fruit possesses. They eat it. They die. They get revived. They later die again. Nobody tries to explore the fruit any further. Brook however, has broken that cycle. He is now doing things with his Devil Fruit, that goes completely against what we were initially told the Devil Fruit can do.
Now lets fast forward back to the present. So in chapter 1037, we see the Gorosei freaking out over a Devil Fruit out of the blue. In the very next chapter, a "Grim Reaper" is in front of Zoro. A very random scene thrown in there, that doesn't really connect to the rest of the chapter. But I think, that scene is meant to connect to the ending of the previous chapter. The Gorosei gets a report that a "Grim Reaper" has appeared in front of Zoro, and all of a sudden, then next chapter, CP-0's orders were changed from capture Robin, to go after ALL of the Straw Hats. Everything is lining up perfectly.
So that brings us to, what is Brooks ability, and why is he standing in front of Zoro, looking like he's about to attack him. This is going to be a lot of speculating, but again, it fits. So one of the abilities that we've seen Brook be able to use since post timeskip, is the ability to be able to manipulate his own soul. But what if that kind of soul manipulation doesn't just stop at his own soul? What if he can see, and even manipulate the souls of other people? Now I know this borders on the edge of the exact same thing of Big Mom's devil fruit, but we also have to remember, that there are many fruits out there that bare similarities, that Oda has confirmed they are more so lesser/greater versions of each other. Oda has even already gone out of his way to pit Brook against Big Mom, and showcase that he possesses the ability to actually damage her soul based homies. Something, no one else alive can do.
So what is Brook doing in front of Zoro? Well, I think he is saving his life. I think Zoro is basically at the point where his soul is about to leave his body, and Brook can see it. The image we see of the "Grim Reaper" standing in front of Zoro, isn't actually the "Grim Reaper" nor is it Brook in his physical form. It is Brook in is astral. He's there to actually prevent Zoro's soul from leaving his body.
We already know souls leave their bodies once a person dies. We know Brooks devil fruit shows us a soul can be put back into a body, even after death occurs. So why can't the man whose own soul left his body after death, and then re-entered it, do the same for others? This also fills in all those "Death flag" theories that people have been talking about for Zoro for years. In essence, Zoro will have "died" but Brook will have brought his soul back to his body.
It all fits. Way to perfectly to be honest. That would only leave the question as to what else Brook can do with his Devil Fruit, and what is the devil fruit's name?
