[yaluckyboy]

I assume that every one who reads the manga know who Brook is

With that said, I don't think that his only powers would be mediocre swordsmanship. I think he should be able to manipulate his bones like Kimimaro Kaguya from Naruto. Due to his body being only bones because of his Yami Yami no Mi, his powers should be allowing him to grow more bones and use them as weapons and eventualy turn into a upgraded form like Kiminaro in Level Two Curse Seal

This ability would be called Hone no Shihai (Bone Control) it allows Brook to control and create bones
Some of his common abilities would be Hone no Yaiba (Bone Sword) where he would do basic Close-Ranged combat, Hone no Hoippu (Bone Whip) which would be his Mid-Ranged, Hone no Taihou Komakai (Small Bone Blast) would be his prefered Long-Range attack since it would be faster and Hone no Taihou Ookii (Big Bone Blast) would be his stronger, but slower, Long-Ranged attack

Later on he would develope Hone no Shihai: Yajuu Katachi (Bone Control: Monster Form) turns Brook into a monster-like form that he can control to do even more damage
His new abilities would be Hone no Tsume (Bone Claw) a new Close-Ranged attack that can be used in multiple and consecutive strikes and Hone no Shouhi (Bone Consumption) where Brook traps his enemy in his ribcage and seamingly consumes him/her (he doesn't really consume them, it's just supposed to look like it)