1000% agree with this. Look at the architecture of Castle Pangea: 
	
	
		
		
	
	
The doors are so big, HARALD FUCKING FIT IN THIS PLACE
The Ancient Kingdom built Pangea, IMO: this explains why people of all sizes are able to fit in the castle. 
It was built to welcome EVERYONE, it was built by the Ancient Kingdom.
The Earth God from the 1st World was turned into a Devil Fruit, like Nika was turned into the Devil Fruit Joy Boy ate, since we know the Ancient Kingdom created Devil Fruits. 
IMO, Devil Fruits were created to recreate powers ancient beings had possessed, that were lost in the war between Sun God Nika and Earth God: we see remnants of this ancient war in Elbaph, with the 3000 year old lab. This proves the 1st World Nika and the Earth God lived in also had advanced technology. 
	
	
		
		
	
	
	
	
		
		
	
	
We know from Vegapunk you can use Lineage Factor to make new Devil Fruits, from Vegapunk semi-cloning Kaido's Fruit, as well as Senor Pink's, Mr. 1's, etc. 
The Ancient Kingdom grave-robbed the graves of the likes of the Earth God/Nika, and used their Lineage Factor to "become" Gods. 
However, we know what happens when Zoan Awakenings go bad. 
Once Mythical Zoans began Awakening their Fruits, it became ABSOLUTE CHAOS.
The "Gods" who Nika fought in the first age, were most likely the Gods Whitebeard described living on the Red Line: the Ancient Kingdom, who ate the "Mythical Zoans" and became Gods.
	
	
		
		
	
	
	
	
		
		
	
	
The "Land of Gods" was the Ancient Kingdom, ruled by those who had used Lineage Factor to create Devil Fruits allowing them to become as powerful as Gods, effectively. 
My one question is where do the Lunarians fit in. 
Birkans from the Moon, who became Cyborgs? 
Where the Lunarians the Ancient Kingdoms attempts, at cloning... actual Angels, from the time of Nika, the 1st World? 
The Halley depicts being with Halos over their heads losing them, and the name Seraphim is completely Angel-based:
	
	
		
		
	
	
There's an Angel next to the Noah, as well, on the right side of the mural. However, unlike the Lunarian on the left side, this angel is white. Is this depicting the Angels the Lunarians were created using the Lineage Factor of?
The Halley is not depicting the 3rd Age at all, IMO: how could they? They hadn't lived it, yet. But, 2nd Age pre-Void Century Elbaph, due to the age of Giants, most likely had records of the 1st Age. 
So, the right of the Hally is the 1st World, and the left is the 2nd World. 
I now believe the Serpent of Hellfire is Pluton, not the Red Line/Jormungandr. Pluton = God of the Underworld, beneath the earth. Imu = Earth God. Serpent of Hellfire = Serpent of Lava, Pluton, which lurks in the Magma of Wano's Mt. Fuji.
What if the Ancient Kingdom WASN'T Ancient to the 1st World, but, to the 2nd as well? WE know Alubarna's Palace was built 4000 years ago, and the Tree of Knowledge was planted 5000 years ago on Ohara. The Ancient Kingdom existed at least 4,000 years ago, in Alabasta.
This means that the war that destroyed Elbaph's history 3000 years ago, did not spread to the rest of the world, and destroy places like Alabasta and Ohara. 
The 1st World depicted in the Halley is the period 3000 years ago. Since we know Elbaph was attacked 3000 years ago, it's likely this Serpent of Hellfire was defeated by the Warriors of Elbaph, stopping it from reaching Alabasta and Ohara, which survived. 
We see Imu in Dragon form and the Serpent of Hellfire breathing fire on a flying ship shooting lightning: Uranus. 
The "Forest God" would be the Sunlight Tree Eve, creating the Devil Fruits "Taming Demons". In Skypiea, the Jayans believed their ancestors' Spirits went into the Sacred Trees of their island. This is why they believed this: the Spirits of the Demons/Gods that died in the 1st World, became the Devil Fruits tamed by the Forest God in the 2nd World: the Devil Fruits. 
So, in the 1st World, actual Gods fought over the world, and destroyed a shit ton of it. 
In the 2nd World, the remnants of the 1st World used the tech from the Ancient Kingdom to research Lineage Factor, and "clone" the old Gods through Devil Fruits. 
We know bad Zoan Awakenings lead to insanity: some of the users of the Devil Fruits went insane when Awakening their Mythical Zoans, especially Nerona Imu, who ate the Demon God Fruit. Man "killed the Sun" by killing Joy Boy, and Poseidon raged. 
The 3rd World has yet to occur, and is not depicted.