Interface, map and ship colours can all be modded to have new ones. For example, the Xitak interface is a strange deep yellow and red theme.
If the mod's done right they will show up when making new races... Not good for multiplayer but free to do/download yourself. If Stardock charged money for what fans could do in a few minutes people would be curious, if not annoyed.
(*cough* which is why the editor is included in the map pack. Sorry SD I love ya
)
However new materials would be cool... And what I would LOVE is new styles. Modded ship sets rule, and have doubled my enjoyment of the game already, but they're really just combinations of what already exists.
So that DLC would be good~