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.