Remember how Madara teleported both Fu and Torune to another dimension.
Right, and we have no idea where they went, then when Madara got his arm infected he repaired it.
Then After that we see The dark side and White side of Zetsu talking and are going to merge and are going to go eat something or someone.
So this is just a theory but I think that Fu and Torune were killed by Madara and in a thank you for Madara's new arm I think that Zetsu is going to eat them or has already ate them. Maybe even ate Danzo as well.
What do you think?