Watching Brook’s Yomi Yomi no Mi develop has been fun to watch.
As we’ve seen, here is the progression:
– Brought Brook back to life, giving him a complete second lifespan (Brook was 38 when he died, so I’ve always taken that as he got a brand new lifespan upon expiring. So he mentally 39 when he came back, but his vitality was reset to being 0).
– From there, he learned that he could leave his body and become a free-floating soul.
– From there, he eventually learned to access the forces of the Underworld. This allowed him to access the freezing winds of Hell. When Brook died, it showed him going to Heaven, so I imagine he can connect there too should he wish. Basically, Brook is serving as a connection between the mortal realm and the realms of the dead and the damned.
– From there, Brook learned that he could use this power to affect the energy of other peoples souls (he basically “killed” the Chess Soldiers in Tottoland using his control of the energy of life and death to excise their mortal soul energy from their bodies, he also cut Zeus, a ball of pure soul energy and flames).
Brook has a pretty strong power set here. His powers are fairly similar to Big Mom’s in the sense that he can effect the vital energy of things around him. Unlike Big Mom, we have not seen him use it on mortal beings, but on animated constructs using harvested vital energy.
So, going by what we’ve seen so far, I do not think that Brook will ever be able to use his powers to effect the living. Big Mom is the soul-based Devil Fruit that holds power over life, Brook is the soul-based Devil Fruit that holds power over death.
This brings me to my point: Brook’s final powers will be to summon the souls of those who have passed on to help him in fights.