Important points in red.
I'm not trying to figure out what Ouranos actually does, but to just establish a foundation of the nature of how the three ancient weapons exist to give a better idea on what they could do.
SECTION 1: ALTITUDES (less important)
Firstly, as you likely know:
Poseidon-god of the sea
Ouranos-god of the sky
Pluto-god of the underworld
Knowing the weapons' namesake gives a correlation 'where' they exist.
The ancient weapon Poseidon was found in fishman island at the bottom of the ocean (a very low altitude).
Ouranos almost certainly will be found in the sky (very high altitude), possibly triggered on the moon. Personally, I think it has to do with harnessing the sun's power as seen in the picture below, but that's besides the point.
Pluto is (or will be) a battleship, meaning it will be at sea-level, which is around the middle of the lowest and highest theoretical altitudes that could be found in the one piece world. Notice how I bolded 'under' in underworld earlier, that was to signify that even though it is the underworld, the fact that in mythology it is still referenced to be very earth-based/within or comprised of earth, making Pluton being sea-level/ground-level still makes sense. Even though I doubt Oda would have thought or cared about something so insignificant.
SECTION 2: EXISTENCE
This is what I really wanted to say:
I've been hearing things about Ouranos being a spaceship and so on so forth, and it doesn't make sense to me. In my opinion, only either one, none, or all three weapons have to be manufactured. It would be strange for two to be built and Poseidon just being the odd man out. So,
Pluton is a built/yet to be built warship. This means it can exist only if artificially created.
Poseidon is an inherited trait passed along living creatures. So its more of an intangible ability passed along through ancestry.
That being the case, I think that Ouranos is both:
1. a force of nature that always exists, and
2. located in the sky
The logical conclusion is that Ouranos is the ability to harness the power of the sun. What that entails is unclear to me (ie. sun death ray, control of night and day, heat waves, etc.) but simply the fact that it may be the sun is important to me nonetheless.