As we know that the Ancient Weapons are Pluton, Poseidon, and Uranus. At first, I thought Uranus is the god of the skies and can control the weather and natural phenomena, but in the light of the recent Chapter 1041, I think it became evident that Uranus is Momonosuke. I will just list the points of why I think so:
– Momo can speak and ORDER Zunisha while Luffy can only hear the voice, similarly to Shirahoshi with the sea kings (Luffy can hear but can’t order them).
– Momonosuke can’t afford to die, before him, Oden said the same thing (I can’t die). This means they have a special attribute inherited within the Kozuki family. There is also Hiyori, but I think it’s only inherited by the eldest son. I can back up this argument with why Toki only sent Momonosuke and not both him and his sister to the future, and also with Oden expecting that “Momo” will be able to talk with Zunisha (Chapter 1041). So its a special attribute only Momo possesses.
– In the Roman mythology, Uranus is a God. Uranus is personification of sky and Momonosuke also start flying by help of his devil fruit. Uranus is also known as father of 12 Titans. And Momonosuke giving order to elephant literally large as a titan.
Momonosuke will bring the dawn to the world by giving the Marines a hard time but the catch is he bring dawn under Luffy’s order and by the help of Luffy.
By this theory, there is one more thing to discuss that there are very high chance Franky still have blueprint of Pluton, and when needed he will tell everyone about it. And then Luffy will have control over all three ancient weapons: Pluton (Franky), Poseidon (Shirahoshi) and Uranus (Momonosuke).
In conclusion, the reason why Momonosuke can’t afford to die is that he is Uranus. He needs to help Luffy, and they will bring the dawn to the world.