So basically, given that Enel showed an interest in the Blue Sea (seen not only when he was speaking to Robin about archaeologists, but also when he asked Luffy what country the Pirate King ruled over) and also that after he was beaten by Luffy he promised to one day return, it’s possible that he will indeed return to the Blue Sea – but the question is — when?
Well, let’s look at Enel’s moon adventure cover story arc… When he entered the ancient city on the moon, he saw two hieroglyphic-like inscriptions.
On the first inscription, we see three winged people on top of a platform, each representing their race: Skypiean, Birkan & Shandian
But something many didn’t notice is that right below the platform (which represents the moon), there’s a fourth person — meaning that there was a FOURTH moon race = a race we haven’t seen before!
And when we compare the two inscriptions, we realise that the Fourth Race left the moon to go to the One Piece World (planet) BEFORE the other Three Winged Races left the moon.
(Here’s the second inscription)
Now, looking at Enel, his head is tilted as if he’s learnt something new — he didn’t know there was a Fourth Moon Race!
I believe that Oda purposely introduced this Fourth Moon Race so that it would be a relevant plot point in the future, which is made likely considering that this is from a cover story and cover stories usually SET UP for a future event (e.g. Jinbe’s cover story linked into the Whole Cake Island Arc).
So, to finally answer the question as to why Enel would leave the moon, I believe that Enel will now strive to look for this Fourth Moon Race’s whereabouts in the Blue Sea.
Alongside him may be his Automata army which he gained the authority of in his cover story.
The identity of the Fourth Race? Well, short answer, I believe that they were the founders of the Ancient Kingdom — which may mean that the D. Clan are the descendants of the Fourth Race since many theories would agree that the D. are the descendants of the Ancient Kingdom.
Speculative part:
If this happens near the end of the series, what if Enel, by a twist of fate, ends up at the location of the Final War.
This links in with my theory that during the Final War, Robin will expose the World Government and reveal the history of the Void Century, while the whole world watches via a Den Den Mushi (like in Marineford).
And knowing that Enel has one of the strongest Mantra (Observation Haki) in the series, what if he overhears Robin speaking about how the D. People are the descendants of the Fourth Race, who founded the Ancient Kingdom and were later destroyed by the 20 Kings.
Imagine, knowing that Enel has a God complex and may want to even become God/Pirate King of the Blue Sea, he decides to fight against the World Government (who, specifically the Celestial Dragons, are the Gods of the One Piece World).
*Theory by Saffron K.
