Attention:Spoilers of episode 31 in here ( not many people may be before this episode (i am on this one) but i don't want to risk)
In this episode when Luffy is with Yosaku and Sanji in their little boat,
Yosaku explains them that the Gran Line has three superpowers and that one of them is the shichibukai (seven great pirates that have allied themselves with the world government).
What are the others superpowers of Grand Line? Akuma no Mi?
Don't tell me only your suppositions because there must official ones.
Link of the video:
P.S.: This is my first post.