I don't know if you've done this, but i suggest you download the game and poke around in the art folder to get an idea about how things work.
Nebulae are a good example of something that we can always use more of. All you have to do is drop a similarly named file in with the other nebula, and the game will use it. (Actually it chooses randomly from the available nebulae, but you can increase your chances by picking a big galaxy and removing a lot of the other nebulae.
I don't know if you can generate sufficient good looking nebulae with your filter creator, but there are always hubble images to use.
These are not yet implemented, but we will need textures to wrap around 3D suns.
You can look in the art folder to see the star colors.