Hey there! This is looking so great! :D Not knowing what I can paint in the BG is something I still struggle with from time to time haha. But since the main focal point here is the deer (besides the camera lens), we don't need to see too much about the BG, just a bit of something to know more about the place where the main FP is. And of course, you don't have to reinvent the wheel all the time. If you struggle with the BG, look for some reference pictures to "copy" part of it. In this case, I took a screenshot from The Lord of the Rings since I like the forest BG and the lighting. It helped me to solve the problem. Don't render the background too much. Just some simple shapes and a bit of texture are enough, but keep it simple since the anelope is the main character here. Hope this helps!
