Simply just that. Folders for organizing layers. As you add more and more layers it would be nice to be able to group them into folders as opposed to just having a list of layers. Just like layers in Photoshop.
Nothing else, no crazy features just a simple collapsible folder to help organize layers.
For my example I have built a open worldy/sanbox-ish VN GUI. Each place in the world is a scene, and each layer in a scene is a location within that scene. For example if one of those places is a house each layer would be a different room/place at that house. As development progresses those layers can start adding up very quickly making for an organizational mess. Which could be eased buy being able to group the layers info folders.
There may be a better way of approaching my project making this less of a wanted or needed feature. However I can’t imagine it would do any real harm either; and who doesn’t love more organization? Anyway this was just a thought as I have been going through and working with Gdevelop 5. Just my two cents. Actually more like loose couch change really, take it or leave it. lol.