This is a list of website we recommend, where you could either get inspiration from, or download assets that could help you push your scenes to the next level.
Every model looks much better with textures, so here are plenty of websites to choose them from:
And some paintings from large art collections:
And if you would like to use prepared models:
And most immersive 3D project really benefit from good audio, so here is a place to start from:
If you'd like to create a visualisation project, based on data, those APIs could potential help you:
If you are after more html/css ideas:
Learn more about webgl:
Since Polygonjs is based on Threejs, you may want to dig deeper if you'd like to create your own nodes and plugins.