Brook’s Devil Fruit grants him a connection to the underworld (he uses the cold from the underworld for attacks and such) and allows him to affect the souls of others (such as his effect on Big Mom’s Homies).
What if Brook could combine those two abilities, and be able to manipulate/communicate with souls from the underworld? If so, then a very heart-warming end of series moment could happen.
We know that Haki Awakening can occur in moments of great emotional/mental stress, like Coby and Luffy at Marineford, or Usopp on Dressrosa. These have been mostly negative, stressful moments, but what if huge, positive moments could have the same effect, and what if DevilFruit Awakening could be triggered the same way Haki could?
All of this is a stretch, I know, but if all these ‘what ifs’ aligned, then we could get the following scenario: Brook finally reunites with Laboon, and he’s so overwhelmed with emotions that his strong feelings reach the souls of the Rumbar Pirates in the underworld.
Their souls manifest before Laboon so the entire crew can be present at the reunion and fulfill the promise. Brook and the Rumbar Pirates’ souls all sing Binks’ Sake along with Laboon’s wailing, everyone crying tears of joy. It’s just an idea I had that I think would be cool to see.
*Theory by aiyhtan