Atleast Momonosuke being pluton theory can be put to rest now. Pluton is all but confirmed as a ship. So what's Uranus gonna be?
IMO, the Ancient Weapons represent the forces that make up the Ocean, and give their users the ability to manipulate it, and rule the seas. So, you have an Ancient Weapon, and a sovereign to control it.
Momo is shown to control Zou: a land beast.
Shirohoshi controls the Sea Kings, sea beasts. So, IMO:
Pluton/Momo = control over massive land creatures, with what is known as Pluton being the biggest one under Wano, allowing one to control the earth by using their stampedes to make earth quakes and move the earth itself
Poseidon/Shirohoshi = control over massive sea creatures who can alter the ocean's currents
Uranus/Nami = control over the sky itself, allowing one to manipulate the weather.
Nami is the reincarnation of Uranus, and will eventually reclaim the moon from Enel, and use it to control the skies. The moon's cycle create tides and alter ocean currents/weather. If Nami gained control of the moon, she would truly be the god of the skies.