Dream dimension ideas

Hllo everyone! I'm currently making a mod that makes minecraft way harder and tedious than it should be and, as the title says, I need ideas for a brand new dimension - Somnis. It's gonna be a final dimension with a true final boss and I need ideas on what I could add in this dimension

This dimension is a manifestation of endless dreams and nightmares, so the possibilities are infinite. If you have something that you think could fit in this dream aesthetic, I'd like to hear about it!

Ideas can be whatever: an entity, block, item, biome or smth else. Thanks to anyone who decides to help :D