Mesopotamian should just be renamed to the Babylonians if we have to choose 1. There are several reasons I think Babylonians are more suitable:
1.) They are not as old compared to the Sumerians. Because of this… we have more information about them in regards to their gods, culture, and military.
2.) If I recalled correctly, the Babylonians defeated the Akkadians. I would prefer to be in control of a historically stronger civ than a weaker one.
3.) The Babylonians, iirc, existed during the Greek, the Chinese, and the Egyptian power. In a sense, the Babylonians would definitely fit right in with them. The Sumerians would predate even the Egyptians, Greeks, and the Chinese.
The Persians would be a great choice. I wouldnt be surprised if they were the next DLC civ. I can see them being in the story towards the end. They have a ton of beastiary that consist of demons. What better way to end the campaign story with an omen from the demonic Persian harbingers? At one time, they were a polytheistic civilization so the Persian would work!
The Aztecs will likely be in it so I wont comment further. Japanese civ would also be a great addition. We need more Asian mythologies.
The Maoris would be a highly underrated civ. I think given its culture and where they live, they would make the game more appealing visually with its beaches and coconut trees, but also their gods and monsters.
The religion of the Sumerians was basically a precedessor of the religions of later Mesopotamian cultures like Babylon and Assyria. I would just make a unified Mesopotamian mythology, with the three main gods being Enlil (mainly for the Sumerian themselves), Marduk for the Babylonians and Ashur for the Assyrians.
Although when it comes to naming gods and creatures, maybe the Babylonian names could be used, as they are arguably the most representative ones).
This would be my favorite mythology to be added to Retold.