It could be the main protagonist if the story is written in a way that the character manages to control and utilize all the Akatsuki powers effectively. With the combined powers, they would have a vast array of abilities at their disposal, making them extremely formidable.
Well, it could be that there's a plot device like an ancient artifact. This artifact might have the power to draw out and combine the Akatsuki powers. The Akatsuki members themselves might be tricked into contributing their powers to this artifact, and then the protagonist or an antagonist gets hold of it and gains all those combined powers. It's a common trope in fanfiction to use such magical or mysterious items to bring about extraordinary power combinations.