I am working on a lit-rpg novel where the universe is called Arcus, and it has three types of planes of existence: Imminus, Medius, and Summus. Each plane has its own characteristics, inhabitants, and environments.
The Imminus plane is the highest and most powerful plane. There are only nine of them, and they have a level cap of 99. They are also the capital and megalopolis realms of the Arcus universe. They are infinitely expanding and everlasting.
The Medius plane is the middle plane and one of the metropolitan planes of Arcus. They are subordinate planes of the Imminus realms, and each Imminus realm has nine Medius planes under it. They have a level cap of 69. Unlike their parent plane, they are not infinitely expanding but are infinite in size and everlasting.
The Summus plane is the lowest and weakest plane, with innumerable realms under each Medius realm that have a level cap of 39.The planes vary in size, ranging from the scale of a planet to that of a galaxy or a self-contained galaxy-sized realm. The Summus realms are also finite in terms of resources and lifespan. They will eventually deplete their energy and collapse into nothingness, unlike the other two planes, which are infinite and everlasting.
The problem is that I want to make the Summus plane relevant for higher realms in some way and my novel. I don’t want them to be just forgotten or unimportant realms where weaker inhabitants are born and migrate to higher realms after becoming powerful enough, like in cultivation novels. Because of the level cap, people need to migrate to higher-level realms to get more powerful. However, if a character is beyond the level cap and decides to migrate to a lower realm, then his or her level will be reduced to the level of the lower realm and they will become weaker.
So, how do I make them unique or valuable so that higher realms would need or want something from them?