Hi,
you should post your idea here "
https://gothicat-world.com/forum/viewtopic.php?f=53&t=9670", it's a topic where you can share your sceneries ideas.
here is a short translation of your idea if you want to post in french -but you also can post in english, don't be worry for that ^^ - .
"salut! J'aimerai suggérer un fond arc-en-ciel. Ils sont chamarrés et donnent de l'entrain. Un fond qui pourrait aller et s'harmoniser avec pas mal de créatures."
Don't hesitate to give more detail about your ideas, another rainbow scenery could be great
-actually only "Reverie d'Elonia" have rainbows in its design-