I can't upload the image for some odd reason so I'll give you the link to see the theory by yourselves while I translate it. Each paragraph represents a tweet, so read it accordingly to see the images. I won't translate the exact same text. I'll skip some worthless parts to get to the point since it's a pretty long thread.
https://mobile.twitter.com/DiamondHakiYT/status/1113418913361080321
"I open this thread to theorize about the ch. 784 cover. This is a short-term theory that might or might not happen in 6 chapters (currently 3), but feel free to add anything to reinforce it.
In first place we need to remember the ch. 520's cover where Oda foreshadowed Ace's death. We can see Luffy holding an ace in one hand while in the other he holds the Death's card, making a similar face to the one he did at MF.
Something similar can be supposed with the 784's cover where we can see some 'winks' of a possible death. Zoro's or Mihawk's deaths have been theorized a lot, but here re the most important details so I can explain to you what I think will happen.
We can see a few skulls which may reference a possible death, some of them are in Zoro's shirt or close to him, like in the zombie's pirate hat. Behind Zoro we see a cross of a grave while in his shirt we see the words 'Último' (last) and 'Mejor' (best).
The first thing I want to note is the grave's cross. There we can see a date: December 20th, a number: 944, and a blurry word. However, when we compare it to the word written behind the cross with another 944 it might say 'Pirates' in it.
Speaking of 944, I'd like to take another example, since I think the 944 references a chapter. In ch. 387 Luffy announced his Gear Second and in ch. 783 he does the same with Gear Fourth. This is an Oda's detail using the same numbers: 3, 8 and 7.
Something similar happened with Nightmare Luffy's first appartion in ch. 478 and Gear Fourth: Boundman in ch. 784. Both shares similar designs and again, Oda's winking to us using the same numbers: 7, 8 and 4.
Okay, in the ch. 784's cover (where coincidentally Boundman makes its debut) we can see a Zombie theme, even with a small detail: 'Picnic with Zombies'. So who is related to Zombies in One Piece?
Exactly, Gekko Moriah. The most curious thing here is that Moriah's debut happened in the ch. 449 in the manga. If we flip the number the result is 944, the number that we can see in the grave. This is why I think Moriah will make his apparition in that chapter.
Let's remember that we have seen him quite recently (ch. 925) at Kurohige's island where the latter offered him to join his crew. We don't know if he's dead or if he's accepted, but do you think Oda'd have show him to kill him later?
Other people may think that he'll kill him to steal his fruit but I don't think so. Kurohige's a smart guy and he knows that Moriah has a huge control over the Kage Kage no Mi, and he might be a quite useful ally. That's why I think Moriah will join the Kurohige's Pirates.
With this, it's more than clear that when the Strawhats will face the Kurohige's Pirates, Brook's battle will be against Moriah as some form or revenge. However the theory doesn't end here.
So, if it's Moriah, why does everything in the cover looks like the 'bad thing' will happen to Zoro? Okay, in ch. 925 Mihawk tells Perona that she leaving the island is the best option since in the Levely a pretty interesting topic has been brought up.
I think that the topic is the Ouka Shichibukai's dissolve. Mihawk might know this and that he'll be target of some pirates, especially Kurohige's. But what does Kurohige gets from killing Mihawk?
In the cover we can see the cross, but we can see another cross in a chest. This might be a clear reference to Mihawk who we know has a special connection with crosses. However, Mihawk isn't going to die. Kurohige wants his power and he knows he won't join his crew.
So capitalizing Moriah's power he'll attempt to steal Mihawk's shadow, but to be able to use it Mihawk can't die, and who's better than Shiliew to use Mihawk's shadow? This way he won't only get a massive power-up but the ability to become invisible as well.
But this would be another reason for Zoro's fight. It's hard to imagine a death battle between Zoro and Mihawk thanks to the relationship between them,but in this case Zoro will fight for his master.
There, after defeating Shiliew with his new powers, Zoro will be the greatest swordsman in the world. I also think it fits perfectly with Mihawk losing his shadow since he lives in a pretty dark place: Kuraigana island, where he could perfectly live there until he gets his shadow back.
There are some details like the date (December 20th) that I don't fully understand if it's making a reference. Maybe Oda believed at that time (ch. 784) that we'll get to that date with ch. 944. So my theor is that in ch. 944 it will be revealed that Moriah will be one of the Kurohige's pirates.
They will go after Mihawk's shadow to give it to Shiliew."
Oof. Like I said, it was a pretty long theory. My only concern is that if all of this is true, Shiliew's powerup won't last too much. Even Luffy only kept the shadows for like ten minutes at best.