Folder structure
Naming
Placing
World Outliner structure
Delete unused content from content browser
General Scene cleanup
Leftover blockout pieces
Unused experimentations
Unnecessary large terrain
Check if everything is on static
Collisions
Delete collisions on some meshes
Add collisions where necessary
Meshes
Particle emitters
God rays
Prefabs
Lighting
Look at settings
Bake lighting
LOD's
Trees
Textures and materials
Texture packing
Check settings
Compression
Combining materials
Culling
Occlusion Culling
Camera Culling
Lighting; the cave area should be dark but it isn't
VFX
Although it probably won't make much of a difference, I want to delete collisions on meshes you can't reach like the ceiling. I also noticed meshes that should have collisions but don't.
The god rays I borrowed from another scene were hardly visible at first. I didn't have any knowledge about how to make it clearer so I just duplicated it a few times. I think because of that, that the shader complexity is quite bad here. I want to delete the duplicated god rays and make sure it's visible in one object. This will also affect the quad-overdraw, which is also going haywire.
The waterfall has quite expensive particle emitters. I plan to have them only load when you're near them. I think that falls under LOD's. I also plan to look at LOD's for the trees.
For folder structure I have only done what was most convenient for me back then. Since UE4 has it's own folder structure, it would be nice to follow that. Not only the structure, but also the naming.
I have a lot of content in the content browser that I don't use. That also deserves a good cleanup.
There are still experiments and blockout pieces in the scene, that don't have a use. I also noticed that the terrain is way bigger than what I use, so I can delete the some terrain pieces as well.
I haven't checked at all how textures are loaded, but I know that the textures for my mask can be compressed which is a lot cleaner. I want to give that a try.
I'll also have a look at if some materials can be instances instead.
My environment is quite big with several regions. I think occlusion and camera culling would fit the best