The hypothesis is Pluton is a flying airship that is powered by Enel’s lightning/electricity power and uses gold as conductor. And Ark Maxim was a mini version of Pluton.
1-For many years, the World Government couldn’t locate the original Pluton and could only try to lay their hands on blueprints held by Tom.
2-We know that Pluton has 2 characteristics. First, Pluton is a ship. The name Pluton has been used to refer to warships throughout our human history as well. In One Piece it has also seem to imply that it is a ship. Second, according to Crocodile, one shot from it will destroy an island instantly. This fits into the name of Pluton as well as it could refer to plutonium used in atomic bombs.
3-Have we seen a one shot destruction from a ship before? The answer is yes. We have seen such a case before in Ark Maxim by Enel when he launches a shot at Angel Island (Raigo). It created a deep crater like hole. Interesting thing is such deep hole has been seen elsewhere too and that’s Enies Lobby which seems too weird to be man made.
4-Ark Maxim has 2 important element to function, it needs electricity and it needs gold to conduct.
5-Enel from Birka, somehow got his hands at lightning devil fruit, went to Skypiea which owns tons of gold from Shandorian and order Skypieans to build Maxim. Enel’s journey is interesting because he seems to know exactly what’s needed to build such ship.
6-Even when Maxim malfunction, he was able to rectify it in one of the scene. The technology behind Maxim, using electricity to create a flying airship, is way beyond what’s seen in blue ocean even exceeding those shipwrights in water seven. Of course, we know that this technology highly likely comes from moon civilization as all elements of building it are by the 3 main races that come from Moon (Birka – Electricity, Shandorian – Gold, Skypiean – Shipbuilding).
7-If the sole purpose of Maxim is to fly back to moon, then it is weird that Maxim is built to produce thunder clouds that could bring about mass destructions with Raigo. All these facts point to the fact that Maxim could have been a model of Pluton albeit a mini version.
8-The name of Maxim, could be traced to Delphic Maxims as a standard for ancient greeks basic morals. One of it says that if one breaks the faith with god, he/she will be punished by Hades. Pluton, a reference to Pluto as well, would in this case refer to Hades too. Hence the link between Maxim and Pluton used by Oda.
9-Gold for some reason is a very important resource. Right after the Void Century, Shandorians were attacked for some reasons. While Shandoria has the Poneglyph revealing Poseidon, that might not be the reason behind this attack. Highly likely, it is for the gold that they own. Because if Pluton was used during the Void Century, then World Government knows that gold is an important resource.
10-Tenryuubito orders for Heavenly Tribute (Tenjoukin) which we are unsure of its contents but in kanji, it is stated as 天上金. If I were to translate it literally, gold from sky. The very reason to control gold resources, could be to control or at least prevent Pluton. Pluton also has an implied meaning to harvest and wealth from underworld. We also know that Ploutos is the god of wealth. Again, drawing the reference between Gold and Pluton.
11-Essentially, Maxim was a ship that works on conversion of energy. Franky, the last person who has seen Pluton’s blueprint, functions as converting coke into energy too. In Maxim, besides giant gears, we do see huge cylinders of liquid that’s being worked as well. Franky was also able to make use of such energy conversion to create jets for sunny go though not to the extent of flying like Maxim but the concepts could be similar. On a side note, was this the reason why Oro Jackson could escape from Shiki?
12-In Jaya Island, when the compass points to Sky Island, Robin was very certain that Sky Island exists. This knowledge could likely be from Poneglyphs and we know in Alabasta, the arc before Skypiea, there’s one that talks about Pluton’s location.
13-Why did Crocodile know and insist that Pluton is in Alabasta? If we trace back to the Poneglyph that Jinbe found and the ancient ruins and the giant sea cat that wears the same earring like Crocodile. Could it be that these ruins are from Birka, the island destroyed by Enel? And in Alabasta, we do see printings of sea cat on the wall design as well. So is Birka existing directly above Alabasta and that Crocodile saw these ruins that Enel destroyed therefore knowing about Pluton? But of course he didn’t know that it is in the sky. Similar to how the World Government doesn’t know that Pluton is in the sky.
14-Back about Enies Lobby, the design is inspired by one island in Oregon USA. The place is called Thor’s Well. Thor, thunder god. Again, relating to Enel and if Enies Lobby is indeed the original destruction by original Pluton, then the design of Enies Lobby reference Thor’s Well would have some sense.
15-To add one more point, I feel that Nami obtaining Zeus is another plot point to the end goal with Pluton and Enel.