To the people saying Brook will learn magic: hasn't Oda said there will never be magic in the OP world? They already have devil fruits and haki and sheer nonsensical awesome to fill that role.
Furthermore there is absolutely zero evidence that the people on that island know the first thing about successfully using magic even if it does exist. If they really COULD use magic, they would have actually summoned something - they didn't. It looks like they pretty much just did the equivalent of killing some chickens and reciting some superstition, and now believe that Kuma teleporting Brook to that island was something they did themselves because they don't know any better.
I've said this before, I know: Brook already has two areas of combat skill, music and swordplay. Making it three would just be messy, character-concept-wise, especially since Oda isn't even done showcasing the musical powers yet. Dragging souls back from the dead for his own benefit seems pretty out of character too, as glad as he was to see his crew buried properly so they could rest. If people had any justification for "Brook will obviously have demon powers" besides "he looks like a skeleton," I might buy it. But Thriller Bark made a pretty clear point that Brook isn't the kind of person he looks like. Oda has written him as a happy guy who loves sunlight and is terrified of the supernatural (and just happened to have had a really horrific accident with his DF power), not some goth demon-powered Grim Reaper.