The Babylonian gods meet. That is the organising image, inherited from Sumer and never abandoned: an assembly of divine powers that convenes, deliberates, and fixes destinies for the coming period. Authority in this pantheon is conferred by that body rather than simply held, which makes it unusually political for a divine order.
Enuma Elish is the clearest statement of how the arrangement could be changed. The younger gods are threatened, the assembly cannot act, and Marduk offers to fight on condition that the assembly grant him supreme and irrevocable authority first. They test him, they agree, he kills Tiamat, and he builds the world from her body. The kingship of the gods is thereby transferred by a vote taken under duress, and the poem exists in part to justify it, because the elevation of Marduk was also the elevation of Babylon.
Membership is correspondingly open. The same text ends by conferring fifty names on Marduk, each of them the name and function of another god, absorbed rather than displaced. When Assyria became dominant, the poem was recopied with Ashur in Marduk's place, which shows exactly how portable the structure was.
Human beings do not enter this pantheon. Mesopotamian anthropology is blunt about the division: people were made to carry the labour the gods were tired of, and immortality, when Gilgamesh goes looking for it, is precisely what he does not get. The one man granted it is granted it as an exception and settled at the edge of the world.