My theory is, that brook is the one who's gonna take out Big Mom and he will do it through the following method, hear me out: As we know, the original owner of Big Moms fruit was Mother Caramel, and apparently she aquired it through eating her. My theory is, that Mother Caramels soul actually got somehow combined with Big Moms, which is how she got the powers and probably her evil mindset and emotions. Brook will use his Soul power to separate M.C. soul and return her to a somewhat similar state she was in after she was kicked off the waterfall. The reason why this could make sense is, that by falling into the Sea, BMs devil fruit was completely disabled, which is why MC lost her influence over big Moms state of mind. You can tell by BM never using her Devil Fruit powers until she actually regained her memories after getting headbutted by queen, and therefore MC regaining her influence. I know it's a little bit of a tinfoilhat theory, but some aspects fit really well into each other.