Maybe it will never be ready for him.
Enels Raigo attack has an identical effect to the strike we saw on Lulusia kingdom with an ancient weapon. Maybe Oda realized that and left him out on purpose. Imagine he spawns a couple of those, rising the seal levels on his own. Without the need for the mother flame and ancient kingdom knowledge.
I would even argue that Enel is better, because he doesn't need to recharge as long. He has a limitless energy source himself, thanks to his devil fruit. That's some broken power if I have ever seen one. (Of course, Oda would nerf his power somehow).
But, I agree. Enel should return. He has a big fanbase.
Enel knows how to construct the Arc Maxim. In Enel's Moon arc, he found an army of Automata/Robot Guys, who are most likely connected to the Ancient Kingdom.
In Enel's Cover Series, he destroyed an Alien Pirate group, who was performing mining excavation on the moon.
This Cover Series ends with Enel revitalizing a Moon city, and the rest of the Robots, by ass-blasting the ancient city with electricity, turning it back "on". Between the mining facilities, robot servants, and electricity powering everything, I'd wager Enel found what was more or less a factory system.
So? Enel now can produce a shit-ton of Arc Maxims.
IMO Enel has at least enough power by now to conquer an entire sea. There are eight conquerable regions in the One Piece world: the Four Blues, Paradise, the New World, the Red Line, and the Ocean itself.
Shanks owns the East Blue, IMO. The Marines own Paradise. Celestial Dragons own the Red Line. Big Mom Pirates and Beast Pirates owned the New World, now, Luffy and Teach are fighting over it.
This leaves us with 3 Blues that have no real established Leader.
I can easily see Enel becoming the "God" of some of this territory, as well as New World territory due to Kaido and Linlin getting beat up by a bunch of rookies.
The Skypieans knew Enel wasn't God, but were forced to follow him out of fear of his power. Dumb fuckers from the surface?
I think Enel could legit become a worshipped God in the near future.
Most likely, though, Enel just beelines to Luffy to beat him up, since Luffy is now, in Enel's eyes, declaring himself as the "Sun God". I'd imagine Enel has SOME knowledge of this (he was shown as being very historically informed in Skypiea), and will be pissed at the very idea of the rubber kid who beat him being worshipped over him.
This leads me to believe Enel is showing up in Elbaph at some point, since this is the first arc exploring the concept of Luffy being known as a "God".