Vivi will use the Voice of All Things to unite the world against the World Government

The Main Theory
I believe Vivi that Vivi's role in the final war will be to round every single faction that we've met and bring them to the battle. I think that she will be able to do this through gaining the Voice of All Things, and that she will use it to communicate a message to the entire world. This power is still a mystery to us and this might seem strange because the Voice of All Things has mainly been portrayed as the ability to hear voices that ordinary people cannot like sea kings or Zunesha. However, I think Vivi's story has been building toward a much more specific application of the ability: allowing her voice to overcome the physical and political barriers that have always prevented her from reaching the people she wants to save. And this idea goes all the way back to the climax of Alabasta.

The problem in Alabasta is that her voice is not loud enough and it is drowned out by the battle. Vivi has the truth, but the people who need to hear that truth cannot hear her. At the end of the battle, when the rain finally falls, Nami notes how "Vivi's voice has reached everyone." I believe that right before the final war, instead of attempting to end a war that the people have been manipulated into, she will ignite a war against the world's manipulators.

I think Oda could eventually repeat this exact progression on a much larger scale. The conflict in Alabasta is essentially a miniature version of the conflict facing the entire world. Crocodile is able to manipulate the kingdom because its people are divided and do not possess the complete truth. The World Government operates on the same principle, except across the entire planet. The people of the world are separated by geography, nations, races, and most importantly, information. The Void Century has been erased, the history of the Ancient Kingdom has been suppressed, and the World Government has spent centuries controlling what the world is allowed to know. Just as Alabasta was divided because its people could not hear the truth, the entire world has been divided because it cannot hear the truth about its own history.

This is why I think Vivi's eventual role could involve communicating with the entire world. Vivi has always been a character whose greatest strength is her ability to bring opposing groups together. That makes the idea of Vivi becoming the person who communicates with the entire world particularly fitting. Luffy may be the person who physically brings down the World Government and liberates the world, but Vivi could be the person who gives the people of that world a voice and allows them to stand together.

The Voice of All Things would be the perfect ability for this role because it is already associated with communication that transcends ordinary human limitations. Roger could hear the voices of the Sea Kings and the Poneglyphs, while other characters such as Momonosuke and Shirahoshi have demonstrated different forms of communication with beings that ordinary people cannot interact with. We don't yet have definitive proof that the Voice of All Things can literally broadcast someone's voice across the entire planet, but I don't think that necessarily makes the theory weaker. In fact, I think Vivi could be the character who demonstrates a new aspect of the ability. The name "Voice of All Things" could ultimately be interpreted not simply as the ability to hear all things, but as a voice capable of reaching all things. Vivi's entire character arc is about overcoming the limitations of communication, so she would be the most thematically appropriate character to reveal that application.

There is also an important connection between Vivi and communication at the very end of Alabasta. When Vivi says goodbye to the Straw Hats, she asks whether they will still consider her their friend. The Straw Hats cannot answer her verbally because doing so would reveal their identities, so they raise their arms and reveal the X marks they had placed on themselves. Vivi immediately understands what they are communicating. They are separated physically, but their relationship survives because their message can transcend ordinary speech. I think this scene is important because One Piece repeatedly treats "voice" as something larger than literal sound. A person's will, feelings, and message can transcend physical distance and even language. Vivi's final role could take that idea to its absolute limit.

The revelations surrounding the Nefertari family make this even more interesting. Vivi is descended from Nefertari D. Lily, one of the figures directly connected to the creation of the World Government and the events surrounding the Void Century. Her family is therefore uniquely connected to the hidden history that the World Government has spent centuries suppressing. Vivi is not simply an Alabasta princess anymore; she is one of the people whose lineage connects the present world directly to the events that created it. If the Voice of All Things is connected to the ancient world, the Poneglyphs, and communication across the boundaries of time and space, Vivi having access to that ability could be another manifestation of her connection to the forgotten history of the world.

Alternate Theory:
I also think Vivi's current connection to Morgans could be important. The girl whose voice once couldn't reach her own kingdom is now associated with the person who has one of the greatest abilities to spread information throughout the world. If Vivi discovers something the World Government desperately wants to suppress, the problem becomes the same one she faced in Alabasta: How do you make everyone hear the truth? Morgans could provide the physical means for information to spread

Ultimately, I think Vivi's story could come full circle through the idea of her voice. In Alabasta, Vivi desperately tries to make her voice reach a divided kingdom, but the chaos of war drowns her out. Once the darkness ends, she finally speaks to the entire nation, repeatedly looking toward the light and the future. The girl whose voice was once unable to reach the people of Alabasta may eventually gain the Voice of All Things and use it to make her voice reach the entire world. Her final strength would be the ability to overcome the very thing that has defined her story from the beginning: the distance separating people from one another. Luffy may bring the Dawn, but Vivi could be the voice that tells the world what that Dawn means.