This theory is about Brook already having Haki and about his future devil fruit awakening.
My evidence for Brook having Observation Haki is this scene is on chapter 849, on page 15, where Brook is in the treasure room and suddenly an enemy comes behind him to attack. We see a weird shine coming from his eyes and after, not only he was able to sense and dodge the attack without looking, but he also managed to attack the enemy, only in a matter of seconds.
Of course, this could have been a lot of things, but it strongly seems to be Haki. Observation Haki has “the power to feel another’s presence to an elevated degree”
and as we saw with that scene with Brook, he was able to sense where the attack was coming from in a blink of an eye. Plus Oda made sure to add this mysterious line coming from his eyes, but we will have to see if the Anime makes it clearer if this is indeed Haki or just another one of his souls abilities. And, from what we have seen, Haki is developed differently depending on the persons fighting style, for example: Usopp is a sharp shooter so his Haki is more adapted to sense peoples presences from a long distance, but Luffy uses more agility and close combat when fighting, so his Observation Haki is more adapted to sensing attacks coming in his direction.
Brook’s Haki could have some different effects, who knows.
Now let me give you my theory on Brook’s Yomi Yomi no Mi’s awakening. In Dressrosa, Doflamingo has his String String fruit awakened, and this is how he describes awakening for paramecia devil fruits: “and when your powers have awakened, it doesn’t only affect you but the things around you as well!!!”
and in chapter 882 we discovered that Katakuri also awakened his devil fruit and is able to turn his surroundings into Mochi.
So based on those two things, I believe Brook’s awakened ability would be to manipulate people’s souls around him, since he already has control over his own soul, making it able to leave his body. The definition of soul from the dictionary is “the emotional part of human nature; the seat of the feelings or sentiments”, so based on this, Brook would be able to manipulate people’s emotions and how they are feeling, probably implementing music into it. Brook actually already has abilities somewhat similar to this supposed awakening, such as using his music to hypnotize his enemies and make them fall asleep. We even had a few moments that could potentially be hints for his awakening, such as when he created that giant soul behind him in Whole Cake Island.
*Theory by Rumbar_29
