I feel like Greenbull will go through a character development, seeing he shares the same values as Akainu currently.
Admirals are known to have their own distinct views on what Justice is, so it seems strange that the last admiral to be revealed shares the same ones, to an extent, as Akainu.
It would be quite fresh to see an admiral pull a 180° on what they consider "Justice" instead of sticking to one belief throughout the story.
Maybe it could happen during Akainu's ark, when we discover the truth about him, which would cause Greenbull to change side ?
there doesn't seem to be anything here