This is probably the dumbest theory that could ever be posted here…
Anyway SPOILERS for chapter 598 (Newest chapter)
! Could Brook and possibly Buggy have obtained "Color of the Conquerer"?
"Whenever someone gains fame in this world they are most likely hiding this power within themselves"
! Now Brook undoubtedly has gotten pretty famous in this 2 years
! And Buggy even got his new pirate crew full of followers and even got promoted (Most likely to Shichibukai)