Brook's Awakened Ability.

So here's my theory...

Brook's devil fruit allowed him to come back to life after life, it also allows him to manipulate his own soul and some other soul based powers

My theory is, What If Brook's awakened power allows him to revive other souls?

He could revive the souls of the Rumbar Pirates, giving him a ghost army.

It would also be a handy ability for when he reunites with Laboon.