Narratively speaking, Oda seems to be enlarging his bigger picture: Luffy turning out to have eaten a mythical fruit of a freedom god, devil fruits being related to the wishes of ancient people and recent mentions to the sea as an entity (the explicit one by Vegapunk but also by Ryokugyu while hyping his power) all draw, as I said, a biggest picture than before.
At this point, I think Imu needs to fit this picture. An incredible haki won't make it, nor a broken devil fruit; that's more of the same. If they're a real person, they need to be something else, either have an inherited will or be alive since the Void Century. But I believe they must be something big enough to encompass the background for the whole One Piece plot and give a feeling of closure once they go down, and to be honest, what I see the most fitting is indeed something related to the sea and opposed to freedom and wishes.
In my opinion, Imu needs to be something directly related to devil fruits, but not have a devil fruit themselves, whose downfall will bring the ending of said fruits. For a proper closure, this story should get rid of the akuma no mi.