We have:
1. Pluton, which is a ship that is capable of massive destruction and
2. Poseidon, which is a mermaid that can control life in the sea, even up to massive sea kings listen to her orders.
Uranus is the god of the sky in greek mythology, so I wouldn't be surprised if it is something related to it. Not to mention that Poseidon is underwater, Pluton is on top of water, so why shouldn't Uranus be something above water, or the skies even?
Since all three are considered ancient "weapons", it could be something that can be used to bring destruction and death. But just like Pluton, which is a ship capable of destruction, doesn't mean it has to be used as such. The same goes for Poseidon: You can control dangerous sea kings with that power, but just like Shirahoshi did you can also use the power to save people (with using the sea kings to stop Noah from crushing FMI).
We have a "living" weapon, and a "constructed" weapon. Maybe Uranus isn't something that needs to be born at a specific time or must be built, but already exists somewhere, maybe it is something natural or even referring to a special place that the ancient kingdom created long ago and can only be activated from someone special, like someone who has a D in its name, or someone who has the voice of all things?
With that said I think it is
1. something that might be capable of flying or is related to the sky
2. that can bring destruction
3. but can also be used for the good
4. something that already exists for a long time and
5. can be activated by only specific people, as well as
6. is connected to the Ancient Kingdom.
I don't think it is a dragon, since we've encountered dragons before and it would be weird if those were just "normal", and then there is one special dragon that is even mightier.
It is just a guess from my side, since some things do point towards it, and that is: Zou (or Zunisha)
Zou has lived and walked for over a thousand years (4), it is a living creature, giant and reaching the skies high above (1). It is a creature uniting above water and underwater. And it can only be heard and commanded by someone who has the VOAT (5). It is an ancient animal and while it does have the ability to cause destruction easily (2, as seen when it attacked Jack and his people). But at the same time it is doing good, providing a peaceful secret place for minks to live (3). It even is a place hiding a Road Poneglyph, which is connected to Ancient Kingdom and the Void Century (6).
Zou fulfills all criteria from my theory, and it has yet to be solved why it has been walking for so long, as it seems to be a punishment. This is now an entire made-up idea, but:
What if the Ancient Kingdom created Zou long ago, be it via experiments or if it is a mutation or such, but it got out of control, maybe even brought destruction, which caused the rest of the world to go against the Ancient Kingdom? And that is why the World Government was created to go against the Ancient Kingdom and destroyed it. Even though the Ancient Kingdom punished Zou by walking the oceans until it's time for that power to come into use. And it could have been used as the Ancient Kingdom fell but Zou was nowhere nearby to save it. Thus it is still waiting to save the world with its mighty powers, as the ancient weapon "Uranus".
