I'm not sure if this is the proper way to submit a wallpaper for an upcoming KDE Plasma version, but I have seen been done in the past here. Anyway here is my creation:
https://imgur.com/a/oz7EpZQ

I call it Autumn Triangles because the triangles are the colors of autumn leaves. Feel free to give feedback and let me know what I should change.

I don't know how to propose it as default desktop for the next version of Plasma, but it's really beautiful. Good job!