Filling a scene with stuff is great but what about the atmosphere between objects?
Make an interesting environment exclusively made of sky, a flat terrain and if you want particle systems. You can use materials scripting, textures, lights and sounds but do not import any 3d asset.
In the example project below you’ll find some particle textures, a script to create skyboxes from a Unity scene, and an example of skydome with a custom shader.
Skies and particles project