It could also happen Fujitora renouncing his position as viceadmiral. All those who have read the manga, know that he does not agree on many aspects with the Marines and the World Government. I expect another fight between Zoro and him, but something tells me he will join the Strawhats in the final war.
I add him to the list of critical voices inside the Marines: together with Smoker, Tashigi, Kobi, General Morgan's son (not sure if he should be here yet), and Aokiji, even if he is not part of the Marines no more. Depending on how the story goes on, even Garp could end here.
But Oda will blow our minds once again with everything. One Piece won't have a generic and predictable ending.