Btw, I have a theory about Sanji's role.
First off, him being with Stussy means that we'll get a bit more history about his lineage, vinsmoke creat, perhaps even the reason why he can use fire. Not to mention, Sanji can read a woman's heart so if Stussy is also hiding something..
Secondly the boots, that lock up. The traitor will definitely locked the strawhat boots but Sanji will be able to use Ifrit to burn through them.
I personally believe that Sanji will be the one to find out who the traitor was.